I’ve revised my Sequel Trilogy concept.
Going against the cautionary advice of his uncle Luke, young Jedi Knight Ben Solo joined this military effort, demanding justice for the lives lost on Mon Calamari. Ben quickly rose to the status of war hero as he became a key player in the battles along the edges of the Unknown Regions. The First Order fought with fanaticism, being pushed back planet by planet at a steep cost to both sides.
Funnily enough, this is similar to what I think should have happened with Anakin in the prequels. Have him pushed into using Dark Side knowledge in a bid to end the war quickly. I kind of see Nolan’s Batman trilogy as a framework. But whereas Bruce is broken by the Joker, Anakin is pushed to fully embrace the Dark Side by a fanatical opponent, and perhaps the mentorship of a supposedly grey Jedi who is secretly a Sith.
Back to the Sequels, I think we have very similar ideas about an Imperial civil war orchestrated by Dark Side agents. A post-Endor Imperial civil war would have given the New Republic the chance to establish itself but also created a different military threat to the galaxy. I always thought the First Order should have been a fanatical splinter group that manipulates it’s way to split or overthrow a more moderate Imperial Remnant.
Over the course of the story I have in mind, I’d have an initially thriving new Jedi order, which is then thrown into chaos. But it is eventually recognised as essential in the fight against a new Sith threat.
Having the Sith Eternal operate as a Spectre-like organisation means you could have an unseen leader, possibly an older generation Sith, and a roster of sinister agents, who are also rivals, you could focus on in different films.