I don't have a party composition yet because the game is still in the planning stages.
I like the idea of Leia turning to the dark side and becoming a sith. What if we throw Mara Jade into the mix and have her train Leia to be an assassin for the Emperor while Vader trains her in the way of the Sith. What kind of story could that lead to?
Frankly, nowhere. You're not writing a novel, you're designing a game session. Unless one of the players is Leia, or they've already met her, there is no reason their characters should care about her the way the players might.
You need to set the stage. You need to create a reason the PCs care about the villains. Leia wouldn't go dark simply from capture. She would need some serious life-changing events. She would also need to fail and become disillusioned. This option IMHO is useless for your players, as you are suddenly not giving them Leia. You're giving them a facade of a character they've never interacted with, and making her act in a way Leia never would.
You might as well ask what events would cause He-man to fall in love with Skeletor and start a band. It's a wierd question, and unless your PCs are He-Man and Skeletor, it has nothing to do with your game.
Start with something you're going to actually use in the first session, based on the Empire winning during "a new hope". I like Tarkin, I like Thrawn, I like Vader, I like Jabba.
I'd start with the basic setting, and an encounter or two. Get the PCs, as this will change *everything* in your campaign planning. What if one of them wants to be a bounty hunter? Or a Jedi? Or a Wookie?
Or none of these things?
I'd say, if they're rebels, have them running from the law. Maybe a mission to steal some capital ships to use as the hidden rebel fleet-on-the-go. Maybe just trying to get the 4-6 of them onto a cargo vessel so they can run.
Their ship could be impounded. Maybe stormtroopers, maybe Jabba the Hutt is collecting on a debt. Both are evil.
Then go from there.
I bet the story will involve running until they stop, or find places that will harbor them, and then they can grow the resistance.
but it's not a book (unless it *is* a book). It's an RPG session.