Super late to this party. I like the ideas people float about on how the movie could've been better so here's mine. I feel the movie was trying to say something different but I don't know why they would even go this route at all since Star Wars is about wars, the Force, and personal choice. TLJ did stuff that made it all sound silly and that's business-suicide when you have such a lucrative franchise with a very understandable expectation from fans.
But I see where they might've been going and here's my thoughts on how it could've been made better.
Note: I am not a fan at all of Star Wars and don't hold any nostalgic feelings or even respect for the lore.
In the Star Wars plot holes thread I mentioned how I don't understand why the Force is held in such reverence. It's a power for good and evil equally and, if anything, is a competing conflict of Order and Chaos. So there's a theme of The Last Jedi where finally the characters question the Force and whether its usage is a good thing or bad. The movie does turn the franchise on its head but there's still one foot in the door in respect to the previous movies that TLJ is afraid of challenging and leaves status quo. It ends up in this awkward position where the arguments are wishy-washy and not given substance. See Captain America: Civil War where a questioning of super-hero adventurism is done well and entertainingly. And so the movie doesn't really say anything and relies on surprises and twists that prey on viewers expecting Empire Strikes Back.
So since one of the original inspirations of Star Wars were chambara films and the Jedi partially influenced by Bushido, I would've said let's just rip off one major theme from Seven Samurai to reinforce the message that I think TLJ was trying to get at. In that film, the samurai are giants but are also the single biggest problem for regular folks and society. Their internal conflicts cause strife and grief on farmers; people trying to make a decent living and just getting on with their lives. But even in this constant abuse, the regular folk persevere to win and the samurai are destined for inevitable irrelevance and doomed to be forgotten. People band together around their work (farming) and a more collective society forms with greater equality.
To reinforce this theme in TLJ wouldn't need a lot of changes to the scenes or the beats; just a tonal shift.
- Luke should have the same grievances with the Force but he shouldn't be in hiding. Let him be more like Kambei: a lone, poor, old, and honor-less samurai (because he cut his hair) yet still uses his skills and Bushido to save the poor and defenseless. Luke is on the rock because he's masterless; his role in the continued "Star Wars" is due to The Force and he believes that regular people have the power to conquer the Sith on their own and he sees his way as done. Ultimately, the Force always gets balanced anyway through major shit-storms (through Anakin, then Luke himself). But he should still be Jedi-like and save little kids and whatnot.
- Rey, as being this super-user of the force, should have some kind of lineage. Doesn't matter which one, just make it one where she belongs to a harmful yet doomed to be irrelevant class which will force her to make a choice. TLJ already has a bit of this notion but goes the opposite way; she's a nobody after all. But I much rather that she really is a somebody: Snoke's kid, Luke's kid, miraculous Force child, etc. because she'd be more interesting to have a theme of choice if she was destined, or doomed, to be a Jedi or a Sith, and whether she even has a choice to decide anything at all. For her the question is not Light Side / Dark Side but rather stepping out of all of that nonsense (Kylo's choice). But as it stands, that choice is less dramatic since there's nothing to tie her to any kind of legacy and so nothing holds her back from making any kind of decision. Kylo is a better character because he does have that lineage.
- How Kylo is set up though, this theme gets lost. Since he's pretty much Vader 2.0 and all the "Grandfather" stuff, he's neck-deep in legacy and his rebellion against Snoke makes sense in a Sith-murder kind of way, or a "is he still able to come back to the Light side" kind of way, not in the way where he rejects any "old" ways. But we need a bad-guy and I don't mind his treatment because he's treacherous and capable but not limited to the same rules that Vader and the Emperor had (namely not having to have a one-on-one lightsaber duel with the main protagonist).
- What would've been really cool is Snoke was really a force ghost all along that whispers in Kylo's ear to make him go crazy. If Yoda, Qui-Gon, Obi-wan, Anakin are all force ghosts, why not some ancient Sith?
- Finn, Poe, and the rest are all filler but are necessary because they are the regular people. They represent what is supposed to be protected. It ain't Jedi. It's Alderaan and their likes from getting blown up by a massive planet-destroying laser. Their failures are actually good because if anybody needs to look like they're in the dark and vulnerable, it's them. But making Leia a force user is ridiculous and betrays the spirit of regular folks, species from all corners, banding together in a rebel alliance. If their leadership is a Jedi or Force user, it doesn't serve the story at all.
- Luke needed to go out fighting with a light-saber. We need to see that he chose to go out as a Jedi because this movie, after all, is called the Last fucking Jedi and that ultimately his choice was to serve the Light side. If the old ways are done, then let it be done through him. He could've even died for very little or even nothing. But he needed to go out by testing the perfection of his skills and for his own personal code and without fear; like a samurai. The force projection is nothing but writers needing a plot device to get past a stuck and make shit up as they go.
Aside from that, I really wouldn't change all that much.