I really like the French Revolution idea quite a lot. It seems to fit well with AC's overall themes, and plus there's a whole ton of juicy subject matter for them to cover. I also used to think the American Revolution would be a good time period to explore, but I'm not quite sure I think that way anymore.
Perhaps just the British Empire in general. It'd be fun to go to different locations other than just Europe. For instance, there's the opium war in China, dealing with pirates in the Caribbean, and the colonization of India and other parts of the globe.
Frankly, there's so much interesting history to cover, I find it hard to believe Assassin's Creed will be able to cover it all, at least not in the main games. Perhaps they'll launch some spinoff games unrelated to Desmond & co. where you play as different assassins in different places/times.