DB is sorely lacking in sympathetic villains, so my idea would be someone kind of like M8 Broly, but with actual loyalty to the "father." Since Vegetto fans get the shaft almost as much as Gohan fans pre-Super Hero, I'd more or less make the climactic battle an homage to the battle between Super Vegetto and Gohan-Boo, where it turns out that Bibidi made another "son" like Babidi, who inherited his previous selves' memories and learned from his mistakes. Instead of trying and failing to sway Fat Boo through sweets or through threats, he creates an incorporeal, magical entity made of dark energy that he gathered throughout the cosmos and the Demon Realm, and infused into it a piece of his consciousness, making the dark, incorporeal entity his "son" so to speak. Goku would then be called upon to investigate the threat by the Kaioshin, which turns out to be a trap: Kaioshin is attacked, and Goku becomes possessed by the dark Majin, although it is incapable of accessing his Blue transformation due to the God form's incompatibility with dark magic. Regardless, Goku's physical parameters are tremendously enhanced, and the Bibidi clone then reveals he chose a time in which Vegeta was away on a solo training journey in order to take out the Blues one at a time and ensure they can't fuse.
Afterwards, the possessed Galu goes on a hunt for Good Boo in order to be complete, but in the process (while defeating Vegeta and then Piccolo in vengeance for Babidi's injury at the latter's hands) the Bibidi clone gets killed by an enraged Gohan, and so he chooses to personally make the Earthlings' lives hell by using the mind control magic to brainwash each wicked individual on Earth to become like Lord Spopovich and wreak havoc. Eventually, Gohan comes to the realization that even if he were to possess Boo, he likely wouldn't be able to achieve his full potential due to Boo's split, so he uses Good Boo as bait to draw him out and then tries to imprison him in the Mafuba bottle, but the dark entity sees through the suspicious nature of the bottle and then reveals he didn't kill Piccolo and instead brainwashed him to startle Gohan before using his consternation to possess Good Boo and reach a solid, near-complete form as his "father" desired. Freed from his possession, Goku goes Blue Kaio-ken and tries to confront him, but even though his hits don't deal that much damage due to his incomplete graduation from his incorporeal nature, his sheer Ki is far above Goku's. In the end, all seems to be lost for Goku, only for it to turn out that Vegeta survived by becoming Majin Vegeta when the dark entity brainwashed the planet, and the two use Kibito's Potara earrings to become Vegetto, this time not wasting time tormenting and taunting the Majin and luring him toward Oob's location before infiltrating his body, destroying the miniature apparition of him (similar to Super Boo's avatar inside himself) and finishing off his incorporeal self with a Final Kamehameha.
I'd flesh it out more but I don't have the time
