Seeing as Desmond is American (I think? He sounds it.) I think the next game, if another ancestor is visited, will probably take place in western Europe/the Americas. My reasoning for this is that Desmond's family would've had to migrate along that path to get him where he is in the present.
If he's in colonial America, then I don't think the free-running will be as... fun. Huts and small wooden buildings don't offer much in that aspect. If in either Western Europe or America during the industrial age, the game would probably have a bit of a 'steam punk' feel with all the locomotives and steam powered crud from back then. Also, the guns wouldn't be so advanced as to give an instant kill, if the assassin is wearing proper armor and all.
I doubt the next game would take place in South America, there's too much foliage. There's also very little motive to move into Asia as well.
I just don't really want a game completely consisting of Desmond the Assassin. AC2 has taught me a lot and I would like AC3 to do the same.
What say you, Gaf?