I'd second Aotrs Commander's concerns about being on the same page as your players. The thing about the Empire, part of the reason it is so popular, is that it looks so cool. Star Destroyers! TIE Fighters! AT-ATs! There is a reason that Kylo Ren and General Hux are basically in-universe Empire cosplayers.

One of the things you might want to consider is to give your characters superiors who won't look cool. Make their commander a weasel aristocrat who only got his post by family connections and have him (or her) steal their glory. Have him nearly get them killed by incompetence and then blame the PCs for his mistakes. Make it clear that a corrupt system like the Empire allows such a toad to achieve a degree of power. Sooner or later your PCs will be itching to get their well deserved revenge.

You might even want to introduce a 'worthy opponent' type to contrast with Commander Weasel. Not necessarily a Rebel but someone opposing the Empire who makes it a point to show he respects the PCs even if he's on the 'wrong' side.