Welcome to the site! I’m getting into writing and hope to write an alternate sequel trilogy one day - in fact I was just thinking about it yesterday so the timing for this topic is great.

In this alternate trilogy, I think I’d keep a couple of things, some of them even things that a lot of people disliked.

Rey is still the protagonist, representative of the new generation to which the torch is passed. She is not a descendant of Palpatine, but from an obscure background as in TLJ and feels unworthy of being the new ‘Skywalker’ figure. In episode IX, I would follow the theme from Colin Trevorrow’s Duel of the Fates where Rey learns that she can be the perfect balance of anger, love, and all human emotions, unlike what the old Jedi orders taught.
Luke is still as depicted in The Last Jedi - self-exiled after his own fear and impulsive mistake caused the destruction of everything he fought to build. However, force ghost Luke is featured a lot more heavily in IX, which includes haunting Kylo Ren.
Palpatine returns as the ‘big bad’ for when Ben Solo turns back to the light, and is how he redeems himself - sacrificing himself while Rey escapes. I like the pulpy way Palpatine returns in Dark Empire, so I’m keeping that part in.

Most major changes would occur in episode 9. Finn, not force-sensitive but trained in lightsaber combat, would lead a stormtrooper revolution. Kylo as supreme leader is unhinged, and Hux - instead of staying a joke character after TLJ - would be so dissatisfied with his leadership that he contacts the Sith Eternal to resurrect Palpatine through a clone body. When this is revealed, the First Order splits into two factions, Hux’s faction being led by Palpatine and the smaller group loyal to Kylo Ren later being mostly inspired by Finn, or dispersing when Kylo abandons his post as supreme leader to fight Palpatine head-on.

I would also have a scene early-on in episode 9 where Anakin’s ghost visits Kylo just before he undertakes a large quest for more power (this would probably be something similar to his quest in DOTF), and warns of the dangers of the path he is taking; Kylo would become angry and tell Anakin that he has finally become stronger than him. Anakin fails to have much of an effect on Kylo, but like in TROS it is Leia reaching out to him that does the trick.

This obviously isn’t comprehensive, it’s just a few ideas.