There's speculation that this is not the first and last Assassin's Creed game, and that in fact, what you play in this game is something like a flashback, seen from the future. It's based on something one of the people who worked on the game said, she said that the game's story (which is not limited to just this one game) might not take place in only that time frame.
So, lots of things can happen at this point. Or rather, after this point, after Assassin's Creed.
I'm looking forward to the free-roaming gameplay, social dynamics and stealthy, stylish assassinations you'll get to do.
There's this one movie floating around, from one of the gaming events' demonstration of the game (I believe it was this year's E3, or TGS) where Altair chased his target through the streets and over the rooftops. He eventually caught up, did a quick lunge at his target, quickly dug his hand blade in his neck and then had to deal with some of the guards that chased him around. It was pretty cool.
Really looking forward to this game. o/