I read an article about it in GameInformer a credible gaming magazine, and it said it would tie together the fates of Altaïr, Ezio, and Desmond. Also, the previous games in the Assassin's Creed series (soon-to-be saga? We shall see.) Have introduced new mechanics and, of course, tools with which the player gains more and more creative ways to mop the floor with an army of Templars. In Revalations, it will be the hookblade, which will make transportation and fighting much more fluent, fun, and fast.
I think they need to take the series to post-WWII France, when the Allies were hunting down the last of the national socialists (which, of course, could suddenly have close ties with the Templars). And it goes without saying, one may obtain an achievement through jumping off the Eiffel Tower.
EDIT: And yes, there will be multiplayer. With new a few new characters like the Sentinel, Bombadier, Guardian, etc.
EDIT EDIT: I do enjoy Ezio's new Turkish makeover. :o
EDIT EDIT EDIT: He's getting some grey in his beard lol, evidently in Revalations he's going to be over 50 years old. He's gonna be running marathons in his 80s.