This is more of a starting point than a full-fledged idea, but I was thinking that it might have made more sense if the Sith Eternal established themselves as an enemy of the First Order as well as of the Resistance.
One thing that’s always bothered me about TLJ is how its ending doesn’t really lend itself well to being “Part 2 of 3” because of the fact that all of the remaining conflict can be resolved by one event - Kylo being redeemed. Luke says “the war is just beginning” and Rey says they’re going to “build a rebellion” but I feel like if I were seeking to overthrow a regime run by someone whom I knew personally, whom I had a means of communicating with, and who didn’t 100% believe in what he was doing, then I’d probably put all my resources in persuading him to my side and having a peaceful transition of power. The movie may have shown him doing some terrible things, but it never really convinced me that turning him to the Light Side would be hard enough to justify waging a bloody war instead. So it occurred to me - in order for the movie to redeem Ben while keeping him in power (which is presumably, something we’d want to see because of how much “Supreme Leader Kylo” was built up in the Last Jedi), the Resistance and First Order would have to team up against a common enemy. Kylo could spend most of the movie doing the right things for the wrong reasons, and his return to the light could be more smooth and gradual.
What do you think?
I agree, and that’s sort of what my rewrite is about - the Resistance’s plan in Act 1 is to capture and turn Kylo Ren. I don’t think that a dramatically new threat is warranted in the third movie, so they are just up against Hux as the new Supreme Leader who is allied with Thrawn, who gives the First Order some much needed competence.
My first draft of the idea on this forum had Kylo turn at the end of Act 1, but the more I thought about it the more it felt too abrupt, as you say. The second draft has him only begin to turn at the end of Act 1, and all of Act 2 is him realizing that his goal - destroying the Jedi and the Sith - has been undermined by Snoke’s sinister plot throughout the trilogy to become a new Sith spirit through Kylo. Act 3 will have him turn fully against evil while still holding to his conviction that the Jedi and the Sith, as they have been known, must die so that the cycle of conflict can end.