Though it may seem that Dragon Ball Z games have approached the franchise from every possible angle at this point, there's still a wealth of unexplored options that could lead to great titles. Dragon Ball Z: Kakarot took a bold approach at retelling the story of the anime yet again, but its methods made it shine and set it apart from the countless other games that have tried it. Likewise, Dragon Ball FighterZ made yet another fighting game out of the franchise, but managed to do so in a unique way that earned praise from the fighting game community. As great as these games are, they are fundamentally reworks of tried and true game styles, so it's time for something new.
Within the story of Dragon Ball Z, there's no denying that Future Trunks is a fan favorite. His debut at the start of the Android Saga blew fans' minds, as the thought of a new mystery Super Saiyan was incredible to see. After making short work of Frieza, he continues on to be an important character in that saga before returning to his own timeline, but Dragon Ball Super reveals that this wasn't the end of his story, as his timeline wasn't restored like he initially thought it would be.
Dragon Ball Z: Future Trunks
While there's plenty of death in Dragon Ball Z, it is almost always offset by the existence of the Dragon Balls. The death of an important character tends to be impactful in the moment, with Krillin's death sending Goku into a rage and the destruction of Earth in the Buu Saga being a harrowing experience, but there's always a way to reverse these issues. Even if the Earth's Dragon Balls fail, there's a set on Namek as well, so the dark moments in the story aren't too dark.
By contrast, in Future Trunks' world, there are no Dragon Balls on Earth as Piccolo is killed by the Androids. The Dragon Balls on New Namek do exist, but it's implied that the knowledge of their location died with Goku. As a result, every consequence in Future Trunks' timeline holds a lot more weight than anything else in the franchise, and the story beats tend to be much darker in general. This would all work together to create a truly unique Dragon Ball Z game, and there's even a lot of leeway available in filling in the gaps of Future Trunks' story.
Which Sagas Would It Include?
There's a couple of ways that Bandai Namco could go about creating a game like this. One interesting idea is to start things off in much the same way that Dragon Ball Z: Kakarot did, with Goku and the other Z Warriors in the Saiyan Saga. This time, the focus would be more on Gohan as he grows up, but the first two sagas would play out the same. In the Android Saga, things would take a different turn, with Goku getting the heart disease and passing away, followed by the Androids killing the remaining Z Warriors except Gohan and baby Trunks.
From here, the story could focus on Teen Gohan trying to survive in this Post-Android apocalypse and raising baby Trunks. As Trunks grows up and gets stronger, players would the be able to play him as he goes back in time to save the main timeline ad returns to take out the Android threat. Moving on from here, the threats of Majin Buu and Goku Black are still very real, and would likely take the forefront of the story moving forward.
Alternate Future Trunks Timeline
For the sake of creating a more compelling game, there are some changes that could be made to Future Trunks' timeline. In canon, he defeats Babidi and his henchmen before they are able to summon Majin Buu, negating that saga almost entirely. Because Goku is dead, Beerus never comes to Earth in search of the Super Saiyan God, and since the Earth's Dragon Balls are gone, Golden Frieza is never resurrected either. With all that in mind, a few small changes would make for a much more compelling story for Future Trunks that fans would no doubt love to see and play through.
If Trunks was unable to stop Babidi from summoning Majin Buu, there'd be an opportunity to see how the Future Saiyan would have handled that saga on his own. In order to prepare for this, Trunks canonically trained with the Supreme Kai for 10 years according to the Manga, meaning it's possible that he could be strong enough to win the fight. Moving forward, his involvement with the Supreme Kai might be enough to warrant the attention of Beerus, and Future Trunks could unlock Super Saiyan God in much the same way Vegeta did. Finally, it isn't a stretch to say that Frieza's henchmen could eventually locate New Namek, using the Dragon Balls there to revive Frieza who would then be keen on getting revenge on Trunks.
Problems with Future Trunks
Unfortunately, something like this is fairly unlikely. In order to make it work as a full game, a lot of changes would have to be made to the Dragon Ball Z canon, and the notable moments in Future Trunks' story are all tied very closely with those of Goku and Vegeta anyway. As much as some players may like to see a game dedicated to this tragic hero, being it looks like he'll still remain a side character for the time being.
On the bright side, Trunks does feature prominently in the Xenoverse games as the leader of the time patrol, though that is admittedly a different take on his story. Unless Bandai Namco Entertainment was given the OK to take the story off the rails and create something completely non-canon, the likelihood of seeing a Future Trunks-centric game is still very low.