Part 2 - Saiyan Arc
Much would change in the 5 year gap. With his young looks and the fact he was popular with a few girls in heaven as an old man, Gohan would definitely find a wife after his performance at the 23rd TB to earn him some popularity and perhaps Roshi would as well. Although it's tempting to pair Gohan with Lunch for the food pun, it wouldn't make much sense in terms of character chemistry. To avoid just calling her "Gohan's wife" or making up a new character, I'll just say he marries Cynthia (the female instructor from the driving filler episode) and they have a son called Sanban (based on Bansan, Japanese for supper). Goku's son is still called Gohan, so I'll be referring to the first Gohan as Grandpa Gohan from hereon unless context makes it obvious to which Gohan I'm referring. With Kuririn seemingly staying at Kame House, I'd assume he'd be significantly stronger at the start of Z due to training with Prime Roshi. Meanwhile, Grandpa Gohan would encourage Chichi to at least let him teach Gohan some basics of self defence and train his tail, making Gohan far stronger at the time of Raditz's arrival. Perhaps the biggest change, however, would be the gravity room/ship existing. With knowledge of Goku's origins, Gohan would've commissioned Dr. Brief and Bulma to put in work of upgrading Goku's ship and installing a gravity device in case they had to fight off Saiyan invaders in the future, likely at some point after the RRA Arc. Without Kami's ship to use as a base and with the fighters not being strong enough to handle it at that point, however, it would take a while to build and remain unused until after the 23rd TB.
That said, when Raditz arrives, Goku and most of the cast would be far stronger. Roshi would naturally get huge GAINZ and Grandpa Gohan would be quite strong from sparring with Goku from time to time, as would Kuririn from training with Roshi. Sensing he was being outclassed, Piccolo would also train far more intensely. Yamcha would likely remain the same due to seemingly being more preoccupied with his relationship and baseball career. As for Ten and Chaozu, I doubt the former would sit back and be left in the dust, deciding to spar occasionally with Goku and use the gravity chamber.
When Raditz shows up, he encounters Piccolo and prepares to fight him. The difference here is that rather than being at 322, Piccolo's weighted self is already at over 1,000. Raditz holds the advantage until Piccolo removes his weights, Raditz then retreating when scanning another high battle power on the move and not wanting to wear himself out fighting the Namekian. He indeed heads to Kame House, where Grandpa Gohan, Tenshinhan and Chaozu are also present for the reunion (maybe Lunch too). The interaction is far different from in canon, with everyone immediately recognising that Raditz is a Saiyan and he is the one who's shocked by this revelation. It's also worth noting Goku would also still have his tail due to knowing about it and simply taking precautions never to view a full moon again. Kuririn walks up to Raditz, this time telling him not to underestimate Earthlings that have had time to prepare for someone like him. Raditz smacks him with his tail, but Kuririn only stumbles back slightly rather than being knocked through the house. Raditz uses his scouter and sees that the Earthling's power is at around 900, much to his shock. Before he can read those of everyone else, Goku steps up to take him on, telling his brother he can either join them, leave Earth or be beaten until he chooses one of the two, removing his weights as he says so. Raditz charges at Goku and its a fairly even fight, with Goku having the edge due to more well honed martial arts skill and Ki control. Raditz is eventually knocked to the ground and before he can counterattack, Grandpa Gohan grabs his tail and makes note of how lacking in skill he is to have not trained his tail. Raditz begs to be let go, which Goku tells his grandfather to comply to. This leads to Raditz knocking him away and shouts to the group that even if they kill him, his two allies will come in one year and each make him look like nothing, flying off after that. Goku decides not to follow him as, if what he says is true, it's more important to prepare for that and Raditz can't do much on his own.
As Raditz heads to his ship to make a retreat, Piccolo tails him and blows up his pod so he can't escape. The fatigued Raditz is at a disadvantage fighting against Piccolo, but causes enough of a distraction with a large Ki blast to retreat. Piccolo wouldn't take too long to find him but, in that time, Raditz relays events to Nappa and Vegeta. He makes note of how impressive it is that Kakarrot has transformed such a pitiful species into a great fighting force and that if one low-class Saiyan is capable of so much, Vegeta could utilise Earth's population to make a grand fighting force. Vegeta finds this interesting and says he'll be there in one year's time. He also tells Raditz that if he cannot subjugate the Earthlings or at least bring Kakarrot to their side before they arrive, he will view him as another traitor and kill the two brothers. After this, Piccolo confronts Raditz after having eavesdropped the conversation. Raditz is willing to accept his death at this point, but Piccolo tells him that he'll need a strong sparring partner if he's to kill two even stronger Saiyans and that it's not as though Raditz is skilled enough to best him (in reality, Piccolo is counting on grabbing Raditz's tail if he tries to kill him). Raditz concedes, seeing this as an opportunity to grow strong enough to defeat both Kakarrot and the Earthlings.
Meanwhile, Goku is contacted by Kami and filled in on the situation. Kami tells him there is a way to assure Goku is fully prepared by going through Kaio's training, but he will have to die first. Goku senses Raditz and Piccolo together, deciding that dying in battle against them is the best option. He goes on alone, though everyone else follows behind in case Piccolo and Raditz use this as an opportunity to conquer Earth. Goku starts to fight Raditz, but then Piccolo attacks both, saying that he doesn't want to kill Goku through someone's help, but killing Goku and getting some sparring with Raditz in will make this a good opportunity for him. This three-way brawl goes on quite a while until Goku purposely goes between a Bakurikimaha from Piccolo and Double Sunday from Raditz, taking the attack head on and dying. It's at this point that the Earthlings arrive. Piccolo notes that Goku's body disappearing must be Kami's doing and mentions the Dragon Balls, to which Vegeta responds from Raditz's scouter that this will make their visit to Earth all the better. At this, Piccolo destroys the scouter. When Gohan sees his father dead, he goes into a great rage and charges at Raditz, knocking the pair of them unconscious. Astonished at his power, Piccolo is intent on training the boy, though the Earthlings stand to fight him. After much arguing on what's best for Gohan and Earth, Grandpa Gohan tries to reason with both sides through a compromise. He deduces Kami will train the Earthlings; to which he proposes Gohan train with Piccolo and Raditz for 6 months whilst the humans train with Kami and the other 6 months will be for all the fighters to train together in the gravity chamber. Piccolo agrees to this before flying off with Raditz and Gohan. Kuririn calls Gohan crazy for this, but the old master notes that this Piccolo is fundamentally different from the pure evil Daimao and someone willing to look at the bigger picture. Gohan still transforms into an Oozaru as does Raditz, with Piccolo destroying the moon, though doesn't remove their tails for obvious reasons.
Thus, the training goes as expected. The humans have all surpassed Kami before this (other than Yamcha and Yajirobe), but still make great gains from sparring with each other and learning the philosophies of Popo's training. Meanwhile, Gohan's wilderness training also consists of Raditz surprise attacking him when not training with Piccolo. Gohan is toughened up far quicker than in canon and moves onto actual sparring with the two, the addition of Raditz's teachings causing him to become far more savage in his fighting yet still emotionally unstable due to his age. After 6 months, they reunite with the humans and start gravity training. All the humans struggle to adapt to 10G, though Raditz finds this easy and Piccolo and Gohan soon get used to it, even moving on to 20G. Meanwhile, Goku reaches Kaio's far faster than in canon, mastering the techniques far quicker. As such, Kaio doesn't forget to factor in the journey back, though Goku still wishes to get in as much training as he can and is revived just around half a day earlier than in canon.
When the Saiyans arrive, Vegeta reads the battle powers of the Dragon Team and notes Raditz has indeed trained them into a great slave army. Raditz, however, is hesitant to deliver on this after having spent months with them and learnt how it is to create true bonds with them rather than living in serfdom. He tells Vegeta that he underestimates how strong they truly are and to prove it. The Saibaimen are grown, though Tenshinhan, Kuririn and Yamcha dispose of them all themselves. Nappa then steps up, to which Piccolo does in kind and Raditz lets Vegeta know for reference that Piccolo is almost as strong as himself. Piccolo then proves himself to be superior to Nappa, though Nappa's durability prolongs the fight. It's at this point that Kuririn notes he can feel Goku's Ki and Vegeta detects a battle power of around 5k, knowing it must be far greater than that in actuality. Even the prince is concerned about the combined might of his opponents and steps in, telling Nappa to deal with the Namekian whilst he takes care of the rest. Vegeta swiftly kills Yamcha and Chaozu, knocking the other humans aside quickly. Raditz and Gohan attempt to fight him but are no match, with Vegeta then seeing Nappa as drawing out the battle and kills Piccolo himself before telling Nappa to deal with the Earthlings whilst he punishes their traitor and his nephew. At Piccolo's death, Gohan gains a rage boost great enough to draw blood from Vegeta but before he can retaliate, Goku arrives. It's at this point that Raditz makes his decision and tells Vegeta he will be neither his nor Freeza's slave no longer and will die in battle alongside his brother if it comes to it. Goku then tells Raditz he'll take on Vegeta somewhere remote whilst Raditz deals with Nappa, giving the remaining Senzu to Kuririn and Tenshinhan who'd be in a fairly critical condition. The fatigued Nappa is quickly brought down by Raditz and comments that, whilst he can't believe a low-class managed to surpass him, perhaps it shows there's hope for the Saiyan race to evolve to the point they can kill Freeza. Raditz avoids landing the killing blow at this point, saying there's hope for Nappa if he joins them. Raditz tells him he'll deal with Vegeta and come back for him later, leaving with the Earthlings to Gizard Wasteland.
At that location, Goku is at an advantage against Vegeta. With his initial strength having been far greater when starting Kaio's training, a regular Kaioken is enough for him to dominate. Vegeta then resorts to using the Power Ball and becomes an Oozaru, though cuts off Goku's tail first; the battle going much as it did in canon as Vegeta wouldn't be fatigued from attempting a beam struggle. Upon Kuririn noting the bright light, Raditz closes his eyes and tells Gohan to do the same. They land and figure out their battle plan. Raditz tells Grandpa Gohan to distract Vegeta whilst Kuririn launches a Kienzan, Roshi using the Bankoku Bikkuri Sho to at least momentarily put him in place. Once it succeeds, he'll remove his and Gohan's tails as they come to assist them. The plan works without a hitch and the Dragon Team defeat Vegeta through their teamwork, though Tenshinhan is killed during the battle. Everyone is resolved to put an end to Vegeta, but Goku objects due to wanting to fight him again. Raditz respects his wishes out of Saiyan Pride, with the others begrudgingly conceding.
In the aftermath, the trip to Namek is decided upon. Gohan, Kuririn, Roshi, Grandpa Gohan and Bulma will take Goku's ship whilst Raditz waits for both Goku and Nappa to heal whilst Dr. Brief makes work on constructing a ship using Nappa's pod. Raditz informs them that it would take Vegeta's pod far longer to reach Namek even assuming that was his destination, but warns them to watch out for any areas controlled by Freeza, telling them about him. The trip isn't performed immediately, with Bulma first having her father spend a few days fine-tuning Goku's ship to have a spot not affected by artificial gravity. Thus, after around 10 days, the group are prepared to head into space. How will they fare? Find out next time...