Imma be honest. I'm not really fond of the "monster from beyond" trope that flies in and is suddenly out to conquer and destroy stuff out of the blue. It's not how conflicts work, or how wars work, and it relies on a fear of "the other" and the "outsider". Star Wars has always been relatively simple as far as its social themes, but it has at least pursued its conflicts from a more human place than that; with the Seperatists being motivated by greed, Palpatine by ambition, and the Empire having elements of real life fascist groups and the First Order reminding one of those who want a return to such ideals. I think having a new group of aliens come out of the blue would be reinforcing negative tropes like "don't trust the outsider", and even if you connected it to "what was already there" you'd have to derive it from something obscure in knowledge to most characters or so long ago it was forgotten... and that isn't really much better.
I think this conversation is kind of dodging the core of the issue. The two examples I mentioned are just examples. I'm pretty sure there must be more ideas that can be used for an antagonist that doesn't involve the complete collapse of RotJ's ending.
Either way though, regarding the external empire idea, I'm not really thinking about a "monster from beyond", but rather something closer to an expansionist empire, like the Europeans suddenly arriving in the new world, but with the "new world" being actually the known galaxy. They could be mostly humans too, just without any kind of force knowledge (aside from maybe some myths they had dismissed as a civilization), but with technology superior to the Republic. So, you'd still get the focus on the Jedi as the key point to save the galaxy, but without immediately writing out the Republic and its army to go back to the "resistance" set up. There would be no need to paint them all as evil, since their own people likely would be oppressed too and by the end of the trilogy you could get defectors from them helping the main cast.
The entire idea is that this could still look close enough to the classic Star Wars for trailers and general visual imagery (facing armies with spaceships, robots, armored soldiers) while avoiding a story reset. The big issue in this case would be the lack of saber duels, since only the Jedi hero(es) would have one, but they likely could replace the enemy's sabers with "close enough" counterparts (like energy blades partially contained within metallic parts or charged metal).
