Co-op seems like the best way to go and while sticking two people into the open-world's we're used to in single-player wouldn't work, we think the infrastructure's already there to accommodate it.
There are two types of co-op, separate from the single-player that we can see working in a challenge room kind of way. First, for some friendly head-scratching, just life the underground tomb concept from the single-player game and make it co-op friendly where a duo of Assassin's has to work together to get from one end to the other.
If killing's your bag then the Borgia Towers in Brotherhood are the perfect ready made arenas for it. Just lift them up, stick them online and whack a timer on there to see which duo can get to the main target the quickest.
In fact, stick a timer on the tomb puzzles as well. Timers for all.
BLAST FROM THE PAST
Then comes an even older assassin. So old, in fact, that he's dead (we assume). Altair is all over the Revelations stuff Ubisoft has been sending out, there's no way he isn't going to feature in the game and potentially heavily.
At least we hope it's heavily. We were tearing our eyes out by the end of the original Assassin's Creed with its crushing repetitiveness but give us some time with Altair with all the new mechanics and level ethos and we could grow to like the chap again.
We've convinced ourselves that Altair is going to be a major factor in Revelations and taking control of him could well work in a similar way to what was seen in Brotherhood with flashbacks and encroaching memories.
Since Ezio is getting closer to Altair's neck of the woods it's almost as if he's following in his predecessor's footsteps. What if we actually get to play as Altair up to the point where he died leaving Ezio to complete some unfinished business in Constantinople?
Remember also that this is called Assassin's Creed: Revelations, which means we're expecting some big twists from the remainder of Altair's fight with the Templars to be unearthed. Here's hoping.