4. Meiji Era Tokyo
Why it'd be awesome:
The Meiji Era was when Japan began to Westernize and open up to the rest of the world. It's entirely plausible that one of Desmond's ancestors could have come to Japan during this period, and this rapid transition between the traditional and the modern would provide an amazing backdrop for any action game. Just imagine: ninja, samurai, and gun-toting soldiers, all fighting together in one big, violent melting pot. Sure, Ubisoft would need to take a few creative liberties with the timeline, but the end result could be spectacular fun.
Why it could happen:
The Meiji Era has all the hallmarks of a great Assassin's Creed story: political intrigue, violent power struggles, and a rich cultural heritage to explore. It wouldn't be entirely unexpected, either: the cryptic messages discovered by Desmond in his cell at the end of the first game contained a few references to Japan.
Why it might not:
While late 19th century Tokyo would certainly lend some amazing atmosphere, it would also be lacking in the way of tall, easy-to-scale buildings. That might sound a bit silly at first, but verticality is crucial to the franchise's gameplay, and Tokyo hasn't historically been home to many tall structures. To make things worse, the broad, sloping roofs that are most commonly associated with traditional Japanese architecture would be a real pain in the ass to climb.
