Call me a conspiracy theorist, but I've noticed that Ubisoft always teases their next settings, very subtly. In AC Brotherhood, we already know they teased III and Unity in the Colosseum vault, and in III, they teased Black Flag with the naval aspect and William Kidd's story. Now they've really been teasing the Far East, from featuring an Asian Templar in Black Flag to Shao Jun, and even to featuring Katanas in Unity and Rogue. And the rope darts.
To add to this, after Syndicate it looks like they will have used all the cool European settings, not to mention the fact that we've been asking for this for I don't know how long! Since we've been to China, and we will already have been to India, I think Japan is pretty much the best option.