One possible plot twist could be that Gohan becomes more powerful than Vegeta much earlier than expected. This would create an interesting dynamic as Vegeta would have to deal with his student surpassing him. Another could be that Gohan discovers a new Saiyan ability that Vegeta was unaware of, which could change their relationship and how they approach training.
A possible plot twist could be that Vegeta Jr is actually an imposter sent to deceive Vegeta. He could initially seem like a legitimate Saiyan but gradually reveal his true motives, which might be related to an enemy's plan to take down Vegeta.
A possible plot twist could be that future Gohan is actually an impostor sent by an enemy to manipulate the real Gohan. Another twist might be that when they meet, their powers combine in an unexpected way that causes some sort of time rift or a change in the timeline.
In a 'Gohan Raised by Vegeta' fanfiction, Gohan's personality would likely change a great deal. Vegeta being his mentor would mean a strict and no - nonsense upbringing. Gohan would probably be more confident in his Saiyan heritage. His fighting style could be a blend of Vegeta's elegance in combat and his own natural potential. Also, the relationship between Gohan and other characters like Piccolo or Chi - Chi would be very different. With Vegeta's influence, he might be more rebellious against Chi - Chi's wishes for him to focus on academics.
A possible plot twist could be that Videl turns out to have some hidden powers herself. It would completely change the dynamic between her and Gohan.
A possible plot twist could be that Gohan discovers a hidden power within himself because of his relationship with Bulma. Maybe her scientific experiments accidentally trigger something in his Saiyan genes. Another twist could be that their marriage is part of a bigger plan by an enemy to disrupt the Z - fighters. The enemy might think that by causing this strange union, they can create chaos among the group.
A plot twist might be that Videl is actually related to one of Gohan's enemies in some way. This would create a huge conflict between them. Also, it could be that there is a prophecy about Gohan and Videl that they are not aware of, and when they find out, it completely changes their relationship. Maybe Gohan loses his powers temporarily, and Videl has to protect him, which is a role reversal from what is expected in the DBZ world.
One possible plot twist could be that RWBY initially believes Salem's motives are good. Since she raised them, they might be blind to her true evil nature at first. For example, Salem could have presented herself as a protector against some other greater threat, making RWBY question their newfound allies when they start to oppose Salem.
One possible plot twist could be that the characters raised by Grimm end up being the ones to save the RWBY world. Despite their upbringing, they develop a sense of justice that is different from the norm. Another could be that they have a hidden power that is related to their Grimm - raising, which gives them an edge over the regular Grimm.
Well, it could be about Gohan achieving some great power-up that makes Vegeta jealous. Maybe Gohan saves the day in a big way while Vegeta fails, which could be seen as 'Vegeta sucks' in the context of this fanfic. As for the 'lemon' part, perhaps there could be some hinted romantic tension between Gohan and another character, but in a very mild, non - explicit way.
One possible plot twist could be that Harry's siren - raised nature makes him a target for dark wizards who want to use his unique abilities for their own gain.