Woah, it's been a while since I've seen anyone mention Geass! I'm afraid I'd be rusty with canon characters for it (and most of those, unfortunately), but the setting really has a lot of potential no matter what. Something that runs concurrent to the storyline might be fun, or that takes place in another location entirely? The whole alternate history of that world is really kinda fantastic.
Though I guess before getting out of hand with suggestions, I should first ask if your interest is more in the day-to-day, the rebelling, the mech battles, the government, or something else entirely.
babble canons are firing at 50% of maximum output, captain!