In keeping with the Flash Gordon origins, I would definitely have Palpatine as the villain. I wouldn’t change Snoke in the first one. Or the First Order. But it needed a tad more politics so we know the state of the Republic. I think I’d have Leia as a Senator as well as leading the Resistance. I would have stuck with the twins from the EU. The girl would be learning politics with Leia and the Boy has fallen to the Dark Side (just as thing start.
From there, big changes. The First Order destroys the Republic Capitol, but the Republic is fighting back as the First Order tries to take over. A year passes to the second movie. Rey goes on a quest for Luke and finds him. He is at first unwilling to teach her, but take Yoda’s message about teaching your failures and have that message be from Leia. THEN you have Yoda and/or Anakin there as Rey gets a more proper training. Ending with Luke accompanying her to save Leia and the others and actually dying in conflict where he shows in person the epic power of a Jedi Master. It turns the tide of the invasion. At the same time hints from the first movie build in this movie that Snoke, only seen as a Hologram, is not who we think he is, but no big reveal, just lots of foreshadowing.
The final movie would be about the final battle. I like the doomsday fleet idea so I’d keep it. I like the fleet of small ships. But it wouldn’t be some impossible to reach location, it would be some far off outter rim secret base and the Republic fleet attacks and is losing until the civilian fleet joins them. Rey faces off with Palpatine and wins. I really love he being backed by the Jedi spirits (Not sure I like some of the fan edit version, there is likely a better way) and I love the 2 saber deflection of his force energy deflecting and feeding back on him making him destroy himself. A very Jedi move that was just perfect.
I like Finn. I’d keep him and most of his story, but Trevorrow’s script had a good bit. I’d have him be more obviously force sensitive and start training with Rey at the start of this film. I’d do more with the defected First Order stormtroopers and they would have some crucial role in the final battle… not on horseback unless it is on the surface of a planet. I liked that bit but to many did not.
The other details I’d have to work out. I’m not sure what other parts I’d have. With the destruction of the Republic Capitol, Leia would become Chancellor. I’m not sure what to do with Han, but having him back out getting in trouble wouldn’t be my line of thinking. I’d have him make some heroic sacrifice that changes the course of things.