Fly the co-op
Ubisoft's own Splinter Cell Conviction proved how addictively fantastic a stealth co-op mode could be, so why not bring that to AC?
While the franchise hasn't always prided itself on a pure-stealth focus, instead making it rather difficult to be a half decent assassin, a co-op mode with improved stealth elements would add a previously unseen level of depth to the online capabilities.
We've already seen a newly implemented cover system that definitely helps out when you're trying to slink around the environments, so hopefully some more stuff is still waiting in the wings to wow us into sneaky submission.
We really like the idea of introducing competitive streaks between the two players, rewarding you for super efficient kills and other 'cool moves' pulled off in play. An arcade style points style reward system could also fill out the experience and leaderboards would keep people coming back for more. We all know how competitive assassins can be.