Just a bit of a speculation thread this one, but I loved AC2, after the disappointment of AC1 (although I loved the Holy Land setting, the game was just too repetitive).
From what I understand, they want to stick to more unusual settings with the game, avoiding the obvious ones. So it gopt me thinking about where and when the next one could be set. Personally I'm leaning towards England during the reformation. London in particular would be an ideal setting, with a mixture of crowded mucky streets and huge impressive buildings (with lots of towers of course!). Edinburgh and Paris might be good too.