Maybe in the fanfiction, Future Trunks time travels to a point earlier than he ever has before. This causes a ripple effect where he has to deal with younger versions of his friends who don't trust him as much as the ones he's used to. He then has to prove his identity and intentions.
An interesting plot could be that when Future Trunks travels back in time, he accidentally brings a small but dangerous creature from his future with him. This creature starts causing chaos in the present timeline, and Trunks has to team up with the Z - fighters to stop it without using his full power so as not to alert the villains too early.
One idea could be Future Trunks bringing back a technology from the future that changes the course of the present timeline's battles. For example, a more advanced scouter that can detect hidden power levels more accurately.
One interesting plot idea could be that Goten and Trunks travel back in time to the Saiyan - Vegeta and Goku's father Bardock's era. They could end up changing some events that lead to a different future for the Saiyans. Maybe they prevent Frieza from destroying Planet Vegeta in a more creative way than expected, like by using some future technology to make the Saiyans seem more powerful than they actually are at that time.
Perhaps a plot where Future Gohan and Future Trunks accidentally travel to an alternate timeline. There, they have to team up with different versions of themselves to defeat a common enemy. This would involve exploring how they interact with these alternate selves and how they adapt to the new environment and challenges.
One idea could be that in a post - apocalyptic future where Trunks is the last hope, Broly somehow appears. Maybe Broly has lost his memory and Trunks tries to train him to use his power for good. Another plot could be a time - travel accident that brings Broly to Trunks' time. They initially fight but then realize they need to team up against a common enemy like a new form of the Androids.
One idea could be that Trunks discovers a new power while on a mission, and Bulma helps him control it in a unique way. Maybe she creates a device that can enhance his power but also has some unforeseen side effects that lead to some comical and heartwarming moments between them.
He becomes more resourceful. In his own time, he's used to a certain set of circumstances. But during time travel, he has to adapt quickly to new situations and find ways to use the resources available in the past. For example, he might have to figure out how to use different senzu beans or power - up techniques that are new to him.
A crossover with the Star Wars universe would be cool. Future Trunks could find himself on a planet in the Star Wars galaxy. Maybe he has to help the Jedi fight against the Sith. His superhuman strength and ki blasts would be a new and powerful element in the Star Wars battles. He could learn about the Force while also teaching the Jedi about ki energy.
One idea could be a crossover with the Dragon Ball Super universe where Future Trunks teams up with Goku Black from an alternate timeline. They could be forced to work together against a new, powerful enemy that threatens both their timelines. Another plot could involve a crossover with the Naruto universe. Future Trunks could end up in the Hidden Leaf Village and learn about chakra. He might then combine his ki powers with some ninja techniques to fight a common foe. A third idea is a crossover with the One Piece world. Future Trunks could land on the Grand Line and join forces with the Straw Hat Pirates. They would have to face off against the Marines and other pirates while also trying to find a way for Trunks to get back to his own world.
A third plot could involve her time traveling to save a member of the Avengers who was crucial to the team but died in her future. She has to figure out how to blend in with the past timeline, deal with the Avengers not believing her at first, and then work together to stop the threat that originally killed that member.
Cell could still be a great antagonist. Imagine a scenario where in the fanfiction, Cell follows Future Trunks through time. He then starts to absorb different beings in the new timeline in a more devious way than in the original story.
One idea could be that Anakin travels to a future where the Jedi Order has been rebuilt in a new and unexpected way. He might initially be confused by the different teachings and practices. For example, the new Jedi might have a more inclusive approach, accepting Force - users from all walks of life, not just those trained from a young age as in his time. This could lead to Anakin having to re - evaluate his own understanding of the Force and his place in the Jedi legacy.