France. 1930s/50s. You as a Cate-Archer-Like cat burglar. Jazz, spies, and thievery! You know you want to steal the Mona Lisa and jump off the Eiffel Tower.
A lot of people were guessing the French Revolution, but I've heard the American Revolution might be as well, which seems a little weird to me, but okay. I'd also love to play in London (or Europe in general! :D) during WWII (imagine creeping around at night, hoping guards don't see you, and then getting bombed!).
Like Chickdigger802 said, Hong Kong/China would be pretty epic, and it would be a bit fun to play as a Ronin. The only problem, of course, would be explaining how you're a descendant of Altair. Then again, it could just be a stand-alone side game. Same with the French Jazz idea.