While I love feudal Japan and think it would make for a cool AC setting, I'd rather they go somewhere uncommonly covered in games and media in general. So Mesoamerica; Mayans, Incans, Aztecs or something like ancient Mesopotamia, Ur, Babylon, etc or like Achaemenid Persia, or India or SE Asia, either ancient empires like Maurya Empire and Khmer Empire or during 19th century colonial periods. Boxer rebellion would also make for a good setting, though China isn't exactly uncommon in media in general and they did have an AC game set there already, albeit a side game. Africa as well is in need of some better representation, with stuff like the Songhai and Mali Empires or like southern Mediterranean Moorish states and conflicts within the Mediterranean, but that's probably a bit too familiar with the series roots and overall Euro-centric nature.