There’s a lot of things I would change. Way too many to list and I don’t exactly have anything fleshed out here nor do I feel like being too long winded, but I think these ideas could have been interesting. At the very least they’d have been better than Rise of Skywalker.
One of the main ones would be to have Finn be more prominent as a character instead of… what they actually did with him in the last two. They set him up as a defected stormtrooper and potential Jedi in Force Awakens and then never did anything interesting with him again. Basically just give him something to do.
Another thing, have Rey be tempted a lot more by the dark side. We got brief glimpses of that in the movies, but there should have been more. Maybe have scenes in Last Jedi of her training with Finn and they serve as mirror images of each other. Finn used to fight with the bad guys and now he’s becoming a Jedi, meanwhile Rey is getting tempted over to the dark side. And then she eventually decides to turn to the dark side and at the same time Kylo sees the light, so him and Finn have to stop her. Then there might actually be some suspense. At the very least they took an actual risk with these characters. Then at the end one of them would kill Snoke and one of them would die and it’d be this big dramatic moment.
Which that’s another thing. Instead of bringing Palpatine back to life, I would keep Snoke alive in Last Jedi and have him be the main villain in Rise of Skywalker.
Another thing I’d change is I’d make the original trilogy characters a little less important to the main plot. Definitely have them be there, but have it be more of a passing of the torch type thing. They already didn’t do a whole lot with Han and Leia, so keep them more as cameos. If you’re gonna keep Luke as a main plot point, I think having him in an Obi Wan or Yoda role is fine.