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.
One plot twist could be that future Gohan has lost his memory due to some strange event in his timeline. So when he meets his younger self, he doesn't remember who he is at first. And as they start to interact, the younger Gohan has to help him regain his memories. Or, it could be that future Gohan brings with him a dark secret from his timeline that could potentially change the entire course of the present - day Gohan's life if revealed.
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 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.
In such a fanfiction, perhaps future Gohan could warn present - day Gohan about upcoming threats that he has faced in his own timeline. He might share his experiences and knowledge, which could make present Gohan a stronger fighter and a wiser person.
One possible plot could be that future Gohan travels back in time to warn the past about an upcoming threat that was not properly dealt with in his own timeline. He might meet the past version of himself and try to train him to be stronger, sharing his knowledge of the future battles. Maybe he also meets the past versions of his friends and family, and has to convince them of the seriousness of the situation without revealing too much about the dark future he comes from.
The plot could also focus on their relationship. Since they are brothers but from different timelines in a way. They could bond over family stories that Goten has heard but Gohan can now give a different perspective on, like stories about their father. And they might train together to become even stronger for the future battles that Gohan foresees.
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 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.
In a fanfiction where Future Gohan meets Goku, it could be an emotional reunion. Gohan might be filled with a sense of relief and joy, seeing his father again. He could show Goku all the things he's been through in the future, and Goku would be both proud and sad about what his son has endured.
Perhaps the biggest plot twist would be if Future Gohan's time - traveling somehow connects to a greater cosmic force or an ancient prophecy. His journey back in time could be part of a larger plan that he wasn't aware of. This could introduce new enemies or allies that are related to this cosmic plan. It might also mean that his actions in the past are not just about saving his own future, but about saving the entire universe from a much greater threat that was yet to be revealed.
In the fanfiction, perhaps they team up to fight a common enemy. Goku's raw power combined with Gohan's strategic mind from the future battles could make them an unbeatable duo. They'd exchange blows and strategies, and their teamwork would be amazing to read about.