The Thrawn Trilogy, maybe condensing and adapting three books into two and then adapting the Hand of Thrawn duology into the third movie. Maybe throw some Jedi Academy/I, Jedi/Kyle Katarn stuff into the mix in the second or third movie.
If I had to use the existing sequel concepts as a base, then I would delete the First Order, delete Starkiller Base, and lean heavily into the angle of Kylo Ren worshiping Darth Vader. The Knights of Ren would be an actual cult of Vader. Their acts would be more like terrorism and less like a military. They would kill and corrupt some of Luke’s students but not all of them, with maybe Rey being their newest student so there’s an audience surrogate for them to explain things to. The Knights’ goal would be more to destabilize the New Republic than conquer or destroy it, and with this they’re pretty successful until the third movie. There are too many competing interests in the New Republic (politicians, planets, corporations, the Jedi, criminals, former imperials, etc.) for it to easily hold together, and they exploit these tensions, probably using assassinations and sabotage, to cause chaos. If Snoke still existed, Snoke would be their leader or maybe their financier. He’s a ruthless alien warlord who got exiled out of the Empire and always hated Palpatine. For a prequel tie in, maybe he’s a former Separatist leader.
Their beliefs would be based on the silly ideas that a lot of current fans have about the Force and how it works, and they would see Anakin/Vader as the embodiment of Balance in the Force, as in someone who was equally attached to the “light side” and the dark side and found total fulfillment by seeking out both (something silly people also say about Revan.) They view themselves as “gray jedi,” with a utilitarian, ends justify the means sort of concept. It would be the task of Luke, Leia (a jedi), and Anakin’s ghost to impress upon them that Vader was enslaved by the dark side and he was not some kind of god of balance, he was a good man who was corrupted by giving into temptation and the power of the Emperor.
It’s up to our heroes Luke, Leia, Han, Chewie, R2 and 3PO, and their pals to defeat the Knights of Ren! Maybe because of the political stuff going on, they have to team up with some scum and villainy types to do it, like a resurrected Boba Fett or some other mercenary guys.