How would you end XXX/Would you change the ending of XXX?
I think some of the most immediate changes I would make would be to the timeline--Luke wouldn't have been on Ahch-To for years and years (or else he would have been searching for something, in addition to feeling like he had to take himself out of the fight), EP8 would have been at least a year out from EP7, and EP9 would have been a few more years as well.
I would have put in more about the political state of the galaxy, would have shown what the New Republic was like, possibly with scenes of Leia being frustrated and still trying to help fix things even if she was no longer a Senator, I would have set up the First Order through her more, shown that she was saying, hey, the remnants of the Empire are still out there, this is a problem that will blow up in our faces if we don't do something. (So, basically, Bloodline, but IN THE ACTUAL MOVIES.)
I would have set up character arcs for the characters better IN THE ACTUAL MOVIES.
Rey's arc is mostly fine, it just needs tweaking. Her journey is the same as every Jedi's before her, one that's a classic for a reason--the galaxy needs the Jedi to rise up again, but it's not an easy journey and she has to face the darkness within herself, she has to confront the fear within herself and let it go, just as all those Jedi Younglings had to do, just as Anakin Skywalker had to do, just as Luke Skywalker had to do, just as Kanan Jarrus and Ezra Bridger had to do, just as Cal Kestis had to do.
I personally would probably nix the Palpatine story (I was a big Rey Skywalker fan for a long time, but the more I settle into SW, the more I like that Luke never needed to have kids) but if that absolutely had to say, I would have hit the notes harder that it wasn't that she actually was dealing with the fear in her heart because she was his granddaughter, but because she herself decided that's why she was feeling fear. Also, Palpatine would not have survived via clones and I would have hit the notes harder that he's Forever Unsatisfied and that trying to achieve immortality through the dark side is not a great idea.
I would have kept the Canto Bight storyline, but I would have added Poe to it, and Finn would have had a storyline about how he was a literal, living, breathing war asset that powerful people decided was just a tool for them to use, so, yeah, he knows what a shitshow capitalism is. I'm not sure what I would have done with Phasma, maybe given her more scenes with Finn to build up tension to the final climax.
We would have gotten Kylo Ren's story IN THE ACTUAL MOVIES, why he turned to the dark side, what reasons he had that he felt that was the choice he wanted to make. I've read the comics and, honestly, even those don't really tell us WHY, other than, maybe? he felt lonely? and Snoke was whispering in his ear? but not why he never felt the need to tell anyone about Snoke? or even what actually happened that night at Luke's school.
I would have made the whole "we need to stay vigilant against the dark side, it's always within us, it's our duty to take a cue from Jedi philosophy, and always confront the dark side that's part of us and work to let it go", like that's always been what GL's whole thing about how the Force works and how the Jedi work and tying that into that people everywhere need to get off their asses and actually pitch in, because you can't depend on one person out of a hundred billion (which is how many Jedi there were at their height) to come save you, you have to be willing to deal with the shit around you and work to overcome it.
Parallel that with Rey's journey to let her fear go, to come to terms with her fear about who she is and who her bio-family is/isn't, that the dark side creeps up on her because Jedi training does make you vulnerable for awhile and have her come to the understanding--especially through finding Luke and Finn--that having other people who can bolster you in the Force, as well as you can bolster them, makes you stronger, so you don't need to cling to others (or the idea