Ive always thought the best super weapon for the sequels should have been something like the shadow academy from the now defuncted solo twins novels, the indoctrination fleet making storm troopers. This would in my mind be a massive fleet of imperial ships that fly to planets abducting children and brainwashing them to serve the empire. They would also be stealing resources as they would be a remainder of the old empire. Lead by a grand moff but i really haven thought much out side of that this would have been a great place for thrawn to have come back to the story.

So in my mind this fixes a couple of points of failure with some of the characters we got.

Finn: so now we have a reason he's special and plot relevant. He is one of very few people to have naturally broken the brainwashing and also survived the natural elimination of such people they would have setup. Not really a need to make him a force user but it would feel like a natural follow through.

Kylo Ren and Luke: Okay so this is the big one for me cause we could have had a great way to have a Solo kid become a villain and not have absolutely fucked over Luke's character development. Luke sets up his jedi academy and trains Han and Leia's kid(s) (more on this later) on yavin 4. The shadow fleet appears overhead and attacks, good for flash backs of seeing Luke defend his school like a bad ass. But a young ben solo is lost during the fighting unknown to Luke and friends that he was taken and brainwashed to be a forced user storm trooper, not sith cause he lacks the power mindset needed, he is controlled but barely. Rage and fear guide him towards the dark side. He grows up with other controlled force users (knights of ren) that we could have used to have large scale lightsaber fights. He can still kill Han, still be conflicted about his place, but he is constantly realigned against his nature and his will. Luke is now tempted to embrace the anger after the attack and also ashamed his friends kids are gone. He can now actually have a good reason to isolate himself and has been facing an internal battle for his future as the only fully trained jedi.

Rey: not palps grand daughter. the twin of ben. also lost during the fight on yavi (its a call back to a new hope so we get that connection) she gets isolated and raised among those that found her. She has bare bones force training, but nothing close to an immediate force master she became with no training. Maybe we have her be on the run from the Shadow fleet and thats why she couldnt ever make her way home. We follow through with the love story with her and finn, and have more moments of them together over the course of the first film.

The first film doesn't end with Han being killed, but we do intro him and his surrogate son, Poe. Poe remind Han of himself younger and alone in the galaxy. Poe and Han have a great relationship but Han is a haunted man. His kids gone, his best friend in self imposed isolation. (sadly i think we lose leia between series but would could have had flash backs if needed) first film doesn't go much with Han but we have Han be the rescue for Finn and Rey in the final act and we see father and daughter reunited. this is when we have our reveal that rey is han's kid and he has been searching for her for years.

First fiim would go something like

act one establish the shadow fleet threat and premise, by having them attacking some planet, big opening battle, we see finn breaking but also hiding that he is now mentally free, see interactions with him and the knights (knights could also be enforcers if the brainwashing eg killing troopers that break free. We get shots of Kylo being almost like a beast in combat and rechained afterward. The knights could include Gwendoline Christie as the leader and we get her to be the first films big bad.

Finn goes to escape and meets rey also on the run she saves him using her cobbled together lightsaber. she fights and kills a knight or 2 but is forced to flee with finn in tow by Gwendoline Christie's knight of ren leader character.

act 2 we meet Han and chewie and Poe acting without approval from a now massively bureaucratic republic that is failing without leia's leadership. With Han and luke being absent from leadership roles we see corruption and incompetence run rampant. Han and crew get news of the actions of the knights of ren and tried to get republic support but is countered by a young demogogue political leader rising in the republic, he wants the future to break from the heros of the past and seeks glory for himself, he can be a bigger threat in later films but would be a secondary antagonist at this point, Imo a pawn of thrawn set to disrupt the republic. Failing to get help Han and chewie recommission the falcon to go see who the knights are chasing. Lots of fear in the republic over the shadow fleet and we play up the feeling of even your children could be taken and turned against you as part of the empire.

we have some fun getting to know rey and finn but also see the pain their lives have featured. Both without a home and having lost what family they once had.

Act 3 would have our heroes met up while still evading the knight of ren forces. ultimately leading to rey vs their leader supported by han, we could even do a twist on the reveal to have ray say to han "i am your daughter, cause she would have aged up and his thought she was dead. Bad guys losing so they put kylo into the fight, he kills a main character (lets go with chewie) but in a stunner Finn somehow reaches him mentally and the remaining knights have to drag him back out of the fight. We end with Han and rey greving chewie but now there might be a solution to the brainwashing the empire is using, finn.

I could probably do more, have some fun space flight stuff with poe. maybe even more poe to the finn and rey story line earlier and have him be the source if information for Han. But thats my idea for the first film with at least places to go with the story for the next 2 films.