I'm with most people in going with the industrial revolution. It would fit with the previous two games, with urban, high-rise environments and that same sense of transition/revolution in both the Holy Land and Italy. Climbing across factory chimneys and warehouses would be cool, but with the rise in primitive revolvers and other guns, perhaps the fighting would become a more static, cover based malarky (which might not fit with the free running elements).
I think that is going to be a forseable problem with the next games: setting it in the future means proper gun battles with automatic weapons, which makes the whole freerunning element far harder to pull off (and make it even less sense to try and sneak up and knife someone in the throat).
Any other locations? I'd hate to see a World War II location, but I wouldn't mind a 20th century setting if they picked somewhere pretty with lots of tall structures, and limited emphasis on gunplay. How about Post-War Prague? Or Riga?