Gohan is pretty darn strong in the manga. He has moments of extreme power, especially when he unlocks his hidden potential. However, his strength isn't always consistent and depends on various plot developments and training.
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 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.
In a fanfiction, they might collaborate in various ways. Firstly, Gohan could offer his knowledge of the Saiyan abilities and fighting techniques. Bulma could then use this knowledge to develop new weapons or defensive mechanisms. Secondly, Bulma could be researching some ancient power sources and Gohan could accompany her to dangerous places to retrieve samples. They would also communicate constantly, with Gohan giving feedback on how Bulma's inventions work in real - combat situations so that she can make improvements.
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.
For Future Gohan, his character could be developed by showing his struggle to maintain hope in a desolate world. He could start to open up more to Bulma about his fears and insecurities. Bulma, on the other hand, could be developed by showing her determination to not let the Androids completely destroy their way of life. She might become more tenacious and less afraid to take risks. Their relationship could be developed as they rely on each other more and more, with Gohan seeing Bulma as more than just a friend who helps him, and Bulma seeing Gohan as a symbol of hope.
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.
Definitely their strong bond. They have been through so much together, so highlighting their loyalty and trust for each other is great. Also, their different fighting styles can add action - packed scenes. For example, Gohan's more methodical and strategic approach against Trunks' more impulsive and aggressive style.