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.
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 the long run, they might realize that they need to change their behavior. They could start thinking about how to make it up to Chichi. Maybe they'll go and do some nice things for her, like getting her some flowers or cleaning the house really well without being asked. This would show that they've learned their lesson and are trying to be better sons.
He might be a bit flustered at first. Gohan is not the most confident when it comes to romance. He could stutter a bit and then take a moment to compose himself before responding positively. He might say something like 'I've always hoped for this, but I'm just so surprised right now' and then go on to express his love for her.
Future Gohan's character development would likely involve him becoming more resolute and determined. After all the hardships he's faced, he would grow stronger both physically and mentally. He might start as a somewhat broken hero, but as the story progresses, he'd regain his confidence and find new purpose in protecting what's left of his world.
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.
He might be surprised at first. After all, having his tail back is a big change. He could start to explore the new sensations and abilities that come with it, like better balance perhaps.
In a fanfiction, Future Gohan and Android 18 could have a relationship based on redemption. Android 18 has done some not - so - great things in the past. Gohan, being the good - hearted Saiyan he is, could see the potential for good in her. He might try to guide her towards a more positive path. This could involve Gohan protecting 18 from other threats, and in return, 18 could use her android powers to help Gohan in his battles. Their relationship could be a slow - burn type, gradually evolving from distrust to a strong friendship or even more, depending on how the fanfiction unfolds.