Alternate Trunks could not return to his timeline?

[withheldforprivacy]

Scenario: After he defeats Mecha Freeza and gives the antidote to Goku, he realizes his time machine has a malfunction and therefore he's stuck in this timeline.

Part 1-Androids Saga
Trunks lives at Capsule Corp, without telling anyone who he is, with Bulma constantly trying to seduce him (she was obviously wet for him in canon). Trunks tries to avoid that, though, eventually, he realizes that he has started getting some... physical reactions to a younger version of Bulma walking around him half naked and touching him inappropriately all the time.
One night, he gets carried away and Bulma successfully seduces him. The next morning, Trunks wants to die of shame, though, later in the day, he decides, in his head, that that woman does not look exactly like the woman he knew in his timeline as his mother and, in a sense, she's not his real mother anyway, therefore it's cool.
Considering how wet Bulma was in canon, all that stuff happens pretty soon after Trunks has come to this timeline, so their daughter, a version of Bra, is born one year earlier than Present Trunks was born in canon. She's born with a power level of 10.000! Trunks is kinda freaked out and inwardly wonders whether that unnaturally high power is a result of some chromosomal anomaly caused by Bra being a product of incest.
As for the training for the Androids, Trunks fails to near Vegeta (no matter what happened with Bulma, he still wants to get to know his father), so he trains with Goku. Having an SSJ as a sparring partner is beneficial for both of them, who become as strong as Goku was in canon in ROSAT right before unlocking ASSJ for the first time (which is a lot stronger than the Androids in my books). Also, this puts more strain on Goku's heart and he gets sick at the 'correct' time, so he takes the medicine and, by the time the Androids appear, he's healthy. Everybody else (Vegeta, Piccolo, Gohan, humans) is as strong as in canon.
Goku easily stomps Nineteen and ends up removing his hands like Vegeta in canon. He spares his life and tells the two Androids to go away, but, that moment, Vegeta, who has been watching everything, comes out and finishes off Nineteen, to Goku's disapproval.
Gero is chased like in canon and escapes thanks to Bulma foolishly appearing. Trunks saves his 'mother with benefits' and daughter. Right after that, he yells at Vegeta for not having tried to save them and says: 'Mom was right about you when she told me not to get my hopes too high.' Because of his outburst, Trunks unintentionally reveals his identity.
Bulma is shocked by the realization that, all this time, she's been getting laid with her son. She bursts into tears and blames Trunks for having never told her. Trunks is red in shame, but this is not the right time for dramas. Gohan takes the upset Bulma back to her house while the rest of the Z Fighters keep chasing Dr Gero.
The other three Androids (17/18/16) wake up. Gero is killed as in canon. Sixteen insists that he is the one to kill Goku, so the two siblings rosambo to decide who fights whom. Seventeen takes Trunks and Eighteen takes Vegeta. Piccolo and the humans are just watching, being of no help.
Trunks neither holds back nor cares about fair play, because of his nightmarish memories of those two Androids, so he takes out his sword and cuts Seventeen into pieces pretty fast. In the meantime, Eighteen and Vegeta have a roughly even fight, since the blondie is not in haste to end things. Sixteen has injured Goku with one well placed blow or two. With a gentle but cold expression, he promises him he will have a quick and painless death if he surrenders, but Goku won't give up.
Trunks intervenes and, with his sword, distracts Sixteen for a while. Eventually, his sword crumbles as in canon. At that point, though, Goku unlocks ASSJ for the first time. He tells Trunks to step back and go to help Vegeta instead.
Trunks kills Eighteen, thus wounding Vegeta's pride. In the meantime, the fight Goku vs Sixteen is roughly even and it all comes down to Goku preparing a full power kamehameha. Sixteen smiles and tells Goku he knows he's bluffing; he'd never put the Earth in danger. However, Goku tricks him with the Instant Transmission and, since Sixteen can't regenerate, he gets killed.
Since Trunks never went back to his timeline, there is no Cell, so the Androids Arc ends here. We'll be back with more action, next time.

 

[Captain Cadaver]

It'd make little sense for someone trying to conseal their identity from Bulma and Vegeta would live right within their vicinity. That said, it'd make far more sense for him to stay with the Son family and Piccolo, training with them and helping Gohan unlock SSJ with what Gohan's future counterpart had taught him. With this addition to their training regime, they would all end up far stronger than in canon and Trunks wouldn't be foolish enough not to make sure Goku doesn't take the vaccine as a precaution. With two Super Saiyans (soon to be joined by three) and no heart virus, #19 and #20 are easily destroyed. Trunks makes note of how they aren't the Artificial Humans of his timeline and eventually uses the information Bulma provides to find Gero's lab, also revealing his identity now that it's safe. The lab is destroyed along with #16, #17 and #18, though the underground lab remains undiscovered.
With the conflict over, Trunks starts living at Capsule Corp. Vegeta becomes far more distant than in the main timeline due to how determined he is to surpass Goku and Trunks. 7 years later, Shin shows up. Piccolo fights against him due to not having Kami's influence within him. Regardless of the outcome, Shin tells him everything and the group head to Babidi's spaceship. They quickly realise how outmatched they are by Dabura, retreating as soon as Dabura kills Kibito; and, thus, decide to use the Rosat. Vegeta and Future Trunks go in first and achieve the Grade forms. Still, they realise how lacking they are against Dabura and choose not to attack yet. Without mastering SSJ, their power would still be lacking to take on Dabura. Goku and Gohan go through the same training as in canon, with the former becoming as strong as his Boo Arc self after the training and the latter surpassing him. Shin is still cautious about attacking Dabura all at once, however, and recommends that Gohan be brought to the Kaioshinkai by Goku. He tells everyone about the Z-Sword and tells Gohan to try and remove it to train with in case the three other Super Saiyans aren't enough. Piccolo also decides to fuse with Kami, though this amounts to nothing.
The group head to Babidi's spaceship, easily killing Pui Pui and Yakon before fighting Dabura. Goku quickly gains the advantage, but Babidi then finds a new target. Vegeta is turned into a Majin midway through the fight and demands to fight Goku. They fight as in canon, the only difference being as Super Saiyans. The two are even, though not powerful enough to revive Boo. Trunks, in the meantime, fights Dabura and is quickly overpowered. Shin dies defending him and Trunks rage at being forced into a helpless situation similar to his future allows him to unlock SS2 and kill Dabura. He then kills Babidi and stops the fight between Goku and Vegeta by force. They then head to New Namek, have Dende become Earth's Kami and use the Dragon Balls to revive those killed by Dabura before heading to the Kaioshinkai. They are amazed to see how strong Gohan has become in so little time and spend some time there training with the Z-Sword, eventually breaking it and allowing Elder Kaioshin to appear. When told of his abilities, Trunks immediately begs to have his potential unlocked, not wanting to be helpless before an enemy again. Elder Kaioshin agrees and does the same for Gohan due to the enormity of his potential. Goku and Vegeta initially refuse, though change their mind after seeing how unbelievably powerful their sons became. Whilst Trunks and Gohan are going through the ritual,

The events of Super progress similarly to in canon. Even without Boo, Beerus still wouldn't just go away when wanting to know about the Super Saiyan God and it'd be far easier to achieve here.
Things go far differently for the Zamasu Arc, however. Going back to Future Trunks' timeline, Cell would've achieved perfection without resistance and Zamasu would still use this timeline for the Zero Mortal Plan. Upon seeing how overwhelmingly powerful Goku Black was, Cell would be shocked, though Black would let him live as he did Trunks and Cell would inevitably become far stronger from his Zenkais and Freeza's cells. It wouldn't take long for him to reach the point where even SSR Black's power isn't enough and the Potara was required. Though outmatched against Merged Zamasu's power and immortality, Cell's regeneration would keep him going until he found an opportunity to escape with the Taiyoken. He'd then use the memories he has of Piccolo to know he could use the Mafuba and attempts mastering that, eventually doing so. For simplicity, I'll say Cell successfully seals Zamasu before wandering the universe, eventually finding Broly and becoming far stronger from the Zenkais gained fighting him.
Meanwhile, the ToP occurs, with Future Trunks taking Freeza's place and the kids being forced to take #17 and #18's places. Without #17's strategy that lead to their victory, however, Universe 7 is defeated by Jiren and erased.

 

[withheldforprivacy]

Part 2-Bra Saga
After recovering from her initial shock, Bulma decides that there is nothing wrong with the situation, so she and Trunks go back to being lovers, though now that his identity is exposed, they treat each other more like mother and son, when they're not in bed. Meanwhile, Trunks keeps trying to approach his father, who eventually becomes less bitter and comes to accept Trunks as his son, not caring that Trunks is the reason he didn't end up himself with Bulma, since, in this timeline, Vegeta never started developing feelings for Bulma in the first place.
As for the training, Goku soon sees the flaws of the grade forms and abandons them. Of course, everybody else follows his example. 5 years later, Goku and Vegeta have mastered the SSJ form, Trunks (who neither trains as hard as them nor slacks off) is on the way to mastering it, humans are as strong as in canon, Piccolo is half as strong as he was in Cell Games in canon (which is, in my books, about as strong as the power Perfect Cell used to tool Vegeta's and Trunkses grade forms), because he hasn't fused with Kami, and Gohan is a nerd who has remained in the low millions range and is barely any stronger than humans.
Anyway, 5 years later, Bra is a precocious child who can easily transform into a Super Saiyan. Sometimes, Goku feels that she hasn't shown them all her power, that her chi feels abnormal, that she might actually be the strongest in the group, though nobody takes him seriously.
Bra often plays with Goten (as strong as in canon) and Vegeta Jr (as strong as Canon Present Trunks). Vegeta Jr is a Saiyan kid who often shows up and looks exactly like Vegeta when he was a kid. He's Vegeta's son, but nobody knows where he lives or who his mother is (in any case, Vegeta keeps living at Capsule Corp). Vegeta Jr mentions his mother sometimes, telling Vegeta that mom is angry that he doesn't visit them often, at which Vegeta scoffs.
Vegeta Jr is a bad influence on Goten and they often mock Bra for being a weirdo. One day, the two boys find out about her being the result of an incestuous relationship and bully her even further. Bra gets mad and her SSJ chi skyrockets, alerting all the Z Fighters on Earth, who fly over to her.
Goku says: 'See? I told you she was stronger than all of us'. Indeed, her SSJ form's true power is greater than any other SSJ's, but her rage doesn't end there. She transforms into SSJ2!
The three adult SSJs (Goku, Vegeta, Trunks) order the two boys to run away and charge at Bra all together. In her SSJ2 state, Bra lacks reason and behaves like Broly. She overpowers the three SSJs and she can actually kill them all instantly, but she chooses to toy with them and torture them.
Finally, Trunks manages to slice her with his sword (saying in his head: 'Sorry, daughter, that hurt me more than it hurt you.'), making her bleed and thus creating an opening for Goku to fire the spirit bomb. Goku manages to form the spirit bomb before Bra recovers, but he is hesitant to fire it, because Bra is just a little child. Fortunately, he won't have to ponder this any further, for Bra passes out because of the toll the first SSJ2 transformation takes on the body (similarly to how Gohan passed out in canon in the anime filler scene in the ROSAT when he transformed into an SSJ2 for the first time).
Vegeta insists that they have to kill her now that they can. Of course, Goku and Trunks object. Goku proposes that they take her in the ROSAT and teach her how to control her emotions. And so, Bra spends two years in the room, one year with Goku and one with Trunks, with whom she has a lot of quality father/daughter moments in there, similarly to Goku/Gohan in canon.
Not wanting to be left behind in power, Vegeta enters too, alone, and uses up all his two years.
Bra calms down, though she still has anger management issues, and life goes back to normal.
Two years later, the 25th Budokai is announced.

 

[withheldforprivacy]

Part 3- Babidi Saga
Having seen Bra's SSJ2, Goku realizes there is a level beyond SSJ and trains to reach it. At the time of the 25th Budokai, Goku is as strong as in canon, except he doesn't have SSJ3, Vegeta is his canon pre majin self and Trunks is as strong as Cell Games Gohan, but, as opposed to Goku and Vegeta, he can become an SSJ2 only when he's angry.
Mr Satan is not a hero that saved the world, but he's still held in high regard as the winner of the last budokai. Even though Gohan does not become the Great Saiyaman or meet Videl (since there is no Satan city), he still participates in the tournament, because, in canon, Chichi said that her father's money had run out, so I guess she'd have made her husband and sons participate anyway.
Krillin is either still alone or married to a nameless woman; either way, it doesn't affect the plot. He doesn't participate in the tournament, since, in canon, he did so only because Eighteen pressed him.
In the Junior Division, Vegeta Jr defeats Goten in the final.
Bra is 12 at this point (counting the two years she spent in the ROSAT), but she's precocious and looks really older, so she manages to trick the organizers into letting her participate in the adult division.
As for the Budokai finals roster, Trunks replaces Eighteen, Bra replaces Mighty Mask and some generic fighter replaces Krillin.
Pintar fights the generic fighter, Piccolo forfeits against Kaioshin and Spoppo trashes Videl worse than in canon. None of the Z Fighters knows Videl yet, so they just show some disapproval of Spoppoviches ways, without being really angry. In the fourth round, Kibito defeats Gohan.
In the fifth round, Trunks is about to humiliate Mr Satan, who has no idea yet how powerful his opponents are. Kaioshin asks him to transform into a Super Saiyan during that fight and the rest of the Z Fighters not to intervene. Trunks shrugs and agrees. He gets on the stage and transforms into a Super Saiyan. Mr Satan considers that a trick.
Before the fight can commence, though, Yamu and Spoppo take Trunkses energy and leave.
Kaioshin and Kibito ask the Z Fighters to follow them. Vegeta does not care (the only reason he came along in canon was that Goku would stay in the living world only for 24 hours), but Bra wants to come along, so she replaces Vegeta and Trunks replaces Gohan.
Dabura kills Kibito and stones Piccolo as in canon.
On the first two floors in Babidi's spaceship, Bra and Goku prisonbitch Puipui and Yakon respectively. On the third floor, SSJ Trunks and Dabura have an even fight, with Trunks trying to tap into his SSJ2 power and end things. Bra is annoyed by her father's poor performance. She declares that she'll go SSJ2 and end the fight herself, but Goku forbids her, reminding her what happened the first and last time she transformed and telling her she's not ready to control that form yet. That conversation reveals Bra's evil nature.
Dabura withdraws and Bra is majinized. She and Trunks are transported by Babidi to fight in the desert. During that fight, Majin Bra shocks her father by revealing to him she hates him! She hates him because, due to him having comitted incest, she was born weird and spent her life bullied and trying to control her anger. Trunks apologizes and, due to his strong emotions, manages to transform into a SSJ2, though he's still outmatched.
Meanwhile, outside of Babidi's spaceship, Goku transforms into SSJ2 and is ready to charge at Dabura, but it's already too late. Buu awakens!

 

[withheldforprivacy]

Part 4- Buu Saga
Bra and Trunks interrupt their fight, having sensed Buu's chi. Bra knocks Trunks unconscious and goes to fight Buu alone. Meanwhile, back outside of Babidi's spaceship, Buu has killed Dabura and is now stomping SSJ2 Goku.
Vegeta shows up, having sensed the monstrous chi, and kills Babidi, annihilating him with a blast. Even with the combined efforts of the two adult SSJ2s, Buu is still too much. Vegeta and Goku end up trapped in a piece of Buu's body and pummeled.
Bra shows up and sends Buu miles away with an off guard kick. She tells the other two Saiyans to leave and that she'll solve herself the problem she has caused. Both Goku and Vegeta realize right away what she means by that. Goku tries to discourage her from doing so, but Bra tells him that she never was happy in her life and she will suffer as long as she lives due to her unstable mentality. With a heavy heart, Goku sees her point and acknowledges it will probably be better this way, so he and Vegeta leave and let Bra sacrifice herself as she has decided. Of course, Buu survives Bra's selfdestruction.
Later, at Kami's lookout, Bulma has an outburst because of Bra's death. She yells at Trunks that she hates him, that it's all his fault, for not having told her in the first place he was his son, back when she was trying to seduce him, that she wishes he had never gone back to the past and it would have been better if everybody had been killed by the Androids.
Kaioshin brings up the Z Sword. Both Vegeta and Goku want to come along, so Shin takes them both to Kais Planet. Vegeta insists on being the first to try to pull the sword and Goku lets him. Vegeta successfully pulls it after transforming into a SSJ. Goku says that since he let Vegeta be the first to pull it, now it is only fair that he is the first to train with it.
Goku and Vegeta start fighting over who should practice with the sword first. In a gag scene, where each of them tries to take the sword away from the other (Kaioshin watching and feeling awkward), the sword accidentally breaks and Old Kaioshin appears.
Old Kai talks to them about his ritual. Once more, Goku and Vegeta fight over who should go first. Old Kai tells them that whoever shows him a photo of a beautiful woman will be the one to go first. Goku shows him a photo of Chichi, but Old Kai doesn't like her that much. Vegeta shows him a photo of Vegeta Jr's mother (we never see that photo). Old Kai says she's gorgeous and agrees to perform his ritual on Vegeta, to Goku's dismay.
Back on Earth, Mr Satan declares he will save humanity from Buu, since he's still blissfully ignorant about the existence of super powers (like in Cell Games in canon). He meets Buu, declares that he has seen through his tricks and tries to defeat him with normal martial arts moves, which, of course, have no effect. Nevertheless, the djinn finds his attempts amusing and grows fond of him, so he and Mr Satan end up being friends as in canon and Buu promises not to do evil ever again.
The gunmen appear and Super Buu is formed the same way as in canon. He goes to Kami's lookout. When he's about to kill them all, Trunks tells him there is a strong guy he can fight. Curious, Super Buu asks him to take him there. Everybody realizes what Trunks is up to. Piccolo tries to discourage him, with a telepathic message, but Trunks is determined; now that the woman he loved, both as a mother and as a lover, hates him, he has nothing to live for.
Trunks takes Buu in the Spirit Room and destroys the entrance, thus trapping both of them in there. Super Buu escapes, turns everybody into cookies and eats them up, except Kami, who escapes the same way as Dende escaped in canon.
Ultimate Vegeta appears and initiates combat. The battle Buu vs Vegeta is even for a while, but, eventually, Buu's stamina starts giving him an advantage.
Old Kai gives Goku the Potara Earrings. Goku teleports to Earth and fuses with Vegeta. Vegerrot kills Buu (in canon, it took Buu forever to consider absorbing him and I don't think, in this case, Vegerrot would let the fight drag on that long).
With the Dragon Balls, Buu's victims are revived (except Bra, whom everybody acknowledges that will be better off dead) and Trunks is taken out of the ROSAT. Bulma forgives him, but, from now on, they're just mother and son, not lovers.
Since it is no longer an emergency, Goku decides not to receive Old Kai's boost but increase his power the honest way instead.