Future Trunks would join in place of Tenshinhan or Roshi in the hopes of restoring his timeline with the Super DBs imo. Since Tenshinhan never actually amounted to anything in the ToP and to cement his place as the weakest link among the Earthling trio
let's say that Trunks replaces Ten and makes victory against Agnilasa far easier, and he is the one to knock himself off with Dyspo through the strategy with Freeza, allowing Gohan to be among the final 5 and allowing the Universe 7 team to win even without #17's fake self-destruction.
Manga-wise, his aid allows Goku and Vegeta to knock off Jiren at the tail end of the ToP following Goku's loss of the MUI form, and #17's wish of restoring the other universes is approved, with Freeza backing down on getting his wish since he knows the team of the other 4 outclasses him. Afterwards, Beerus and Champa, as thanks for restoring the latter and his universe, allows Trunks to have access to the Super Dragon Balls soon after the ToP and wish back his timeline, where he returns to, but most likely after the events of the Broly saga as his presence helps to make Freeza's role as a villain much more interesting there. Nothing much else changes, since Trunks would probably never return to the present timeline.