That’s a cool idea. Honestly you could’ve had something similar to what was already in the films. The First Order could’ve been your Imperial Remnant, fighting with the New Republic, then a Sith Cult out in the Unknown Regions introduces a new threat that brings the two factions together. Maybe they wanted to resurrect a Sith Lord and begin a new Sith Empire. Could’ve been Sidious, or Darth Bane, or a brand new guy.
I’d have made the First Order Luke’s Jedi Order after Kylo Ren fell to the dark side and booted Luke and his loyalists out — a dark side cult/terrorist faction hostile to both the New Republic and Imperial Remnant. They would’ve manipulated events to get the Republicans and Imperials embroiled in a new war, would would’ve destabilized both sides, leaving them vulnerable to the First Order’s true masters — alien invaders from the Unknown Regions.
I like this idea. Anakin destroyed the Sith, but now another faction has taken it’s place. I think this would’ve helped feed into Luke believing the Jedi fundamentally flawed and wanting something new to take its place.
Really the key would’ve been to have made the Knights of Ren bigger antagonists. They could’ve been separate from the Imperial remnant/First Order, but they could’ve used Ben Solo’s status as “Heir to the Empire” to incite attacks against the New Republic. Make the Empire great again and all that. Which would lead to war by the end of the first film.
Kylo Ren could’ve deposed the Supreme Leader (non-Force sensitive, just an Imperial Warlord) at the end of the second film, and declared himself the new Leader/Emperor.
Maybe in the third film, the Knights try to depose Kylo, and he ends up having to work with the good guys to stop their plans, but ends up going good and helping reunify the “Empire” and “Republic”.
Luke could’ve helped train the next generation of Jedi before sacrificing himself, including Rey, Finn, and maybe Poe. And then you could’ve had a few Jedi versus a few Knights of Ren as the climax. But the big thing would’ve been to add characterization to the other Knights and give them their own motivations, too.
I really don’t think the broad strokes would have needed to be much different (besides the last film).