I've seen a few threads about people trying to balance the game for 5 rebels, but never have I read about someone adding another Imperial.

Article from Shut Up & Sit Down.

Let the fifth player choose an Ally deployment card and they can play as them. The 6th player can choose a villian to play as, the imperial player either pays the difference in threat if higher or gains that much threat if lower.

That's a good idea too, though playing the Ally would probably feel really underwhelming compared to all your awesome Hero teammates.

Playing a Villain on the other hand could probably be pretty fun, though that might feel odd in story missions: having a villain show up wherever you go.

That's a good idea too, though playing the Ally would probably feel really underwhelming compared to all your awesome Hero teammates.

Not at all! Han, Luke, and Chewie are all brutal! Chewie is almost impossible to hit at times.