There likely should have been more of a plan for what the whole trilogy would look like from the beginning. It's hard to know exactly how much was actually planned, because we do know the intent was for Leia to confront Ben in this movie (one example), and that still happened with Han being subbed in.
The original trilogy didn't have an ending planned out. That's understandable, in that no one knew if there would even be a single sequel. Still, that ending was mediocre. Jedi had no new story to tell. The first chunk of the movie is just rescuing Han, Luke visits Yoda and nothing much new develops there, and then we're into the finale (with another Death Star no less). That movie nailed the landing with an epic space battle and a satisfying showdown between Luke/Vader/Palpatine, but it doesn't do much else and has a myriad of other issues.
The Last Jedi was short on developing the larger story, but it did do things that set up a finale where we would actually get some story development. The Resistance needing to rebuild and the notion that there are many more force users, with the latter not ultimately touched on. That would have been a good addition to the ending. We see a few First Order ships going down, why not show a few people rising up and using the force to take out some storm troopers for example?
What I'm looking for, is was the overall story where Rey is a Palpatine actually planned? Was it effective? This was a real missed opportunity. From the time we met Rey she was always a good person. In TLJ she "went straight to the dark", but it didn't amount to much. It wasn't until Rise that this story even started. The scene where she killed Chewie was great, we see her using Sith powers out of anger and simultaneously feeling conflict over the terrible act she just committed. Why wasn't there this conflict from the beginning? There was never any mystery or question of whether or not Rey would consider joining Ben and the dark side. I believe going to the dark side for a time was something considered for Luke in RoTJ as well, but abandoned.
From the beginning we were told Rey is nobody, while at the same time dropping a few suggestions there might be more to it. Leia embraces Rey like a daughter in TFA (as noted in the script). Was this because she knew the importance of expressing love and compassion to the heir of Palpatine? When Kylo asks "what girl?" with utmost importance, did he have an inkling there was someone of importance out there? Was Luke's hesitancy to train her due to knowing who she was? These would be interesting if they tied into the larger story but it just requires a lot of inference.
I wish there was more in TFA to setup a more interesting journey for Rey. Even something like using force lightning against Kylo would have set up an internal conflict of good vs evil, and given a clear suggestion that her lineage might come into play. Instead, the heir of Palpatine is a 100% good person all of the time, which is odd, and leaves no suspense at the end. Of course she would never pursue the notion of ruling the galaxy, she's all good all the time.
It's still a fun movie. There's a decent amount of story, humour, chemistry among characters, action, etc. I just wish the Rey story was more than good person grows up to defeat the Emperor. Ben Solo was the one who got the story of good vs. evil, but his story was secondary to Rey's. Why wasn't he more integral to taking down the Emperor at the end of the day?