I don't think they will be going to Japan any time soon. Because that is what I want and no video game company ever does anything I want. Maybe China just to mess with me.
I think the French revolution is a good bet for the next one. Going through the audio logs in ACIV there is also some major stuff happening in Nazi Germany but I'm skeptical that will happen soon.
What about Egypt? A picture was leaked from Ubisoft a while ago, It looked like a next gen Prince of Persia game because it was set in Egypt. Turns out a picture of the same setting was used in ACIV. They even talked about assassins from ancient Egypt in that picture.