I enjoy Future Trunks, and in some ways I wish he'd merge into the main cast (especially in Super as a immigrant from a destroyed reality instead of going back to another reality where there's now two of him), but in other ways I feel he was just sort of... I dunno, redundant? Forgotten? Not utilized?
The History of Trunks did a good job of sort of extending out the world beyond the core team and their effect on the world around them, known or unknown, and how a world might operate without them. It really felt like its own little world and filled out story, and then when Trunks visited the world of the main cast he felt like a true hero of another story intersecting to help save the main universe. Good times all around. However, as it descended into the Cell Saga it sort of felt like he was just being used as a plot tool as the confidence he had beforehand was replaced by a need to appeal to a father he never knew and as a plot device to highlight the differences of what was to be the new Super Saiyan form Gohan would take on as Goku and Vegeta were clearly aspiring for one.
This would be partially rectified in Super where Trunks has basically taken on the old alternate Gohan's mission (and even his story beats) of protecting the world, but would fall into the same exact trap of passive need to impress others with low confidence to again highlight Vegeta and craving Vegeta's approval. He was shown to be a bit trickier here to highlight his need of cunning over brute power, but it was never utilized in any way and then fell into the old Dragon Ball trap of "here's another transformation/form to brute power your way out of things." Ultimately I feel his character was cut at the leg to ensure Vegeta and Goku shine brighter despite the fact he should have been a little bit different and rougher around the edges with a more unsure but confident attitude. Like Gohan, there's just so much potential in his character that never goes unrealized. Granted the series sells on Goku and Vegeta, so it's not like the story can just radically shift to the story of Trunks or Piccolo or Tien or Gohan. But a man can dream. I just wish he'd grow beyond the same exact story beats he gets every time. Shows up, is powerful but maybe not as powerful as the people he needs to warn/learn from, kowtows to Vegeta and trains to master power with him or learns to resolve via his father, then gains a new half form that while powerful is just not enough, therefore buying time for the A team to arrive to solve the fight for him.
Honestly I'd like a little side series, "The Future of Trunks" which extended past his time in the Cell Games where he'd go on to fight some of the threats of his world including Majin Buu instead of having it all explained in brief. Because Future Trunks is easily one of the more interesting characters in the series who never had the benefit of formal training beyond a couple of years and had to do everything solo as the only powered fighter (beyond Roshi) on Earth. His struggles wouldn't be simply, "Find new form/use new form/fuse/unleash ultimate power," the same way. He'd have to be a lot craftier, and I'd have loved seeing that story. And if he made it into Super, it would've been a great bit where he had to train present Gohan in a reversal of master/student to catch him up for tournaments or whatever.
