A report on website TGL claims to have screengrabs of a survey currently being conducted by Ubisoft, sounding out people's thoughts on the future of the Assassin's Creed franchise.
The two pertinent questions they've posted give an insight into things Ubisoft is obviously thinking about. The first is asking whether people would be into the idea of another player jumping into your singleplayer campaign to give you a hand. The second is seeing how interested you'd be in another Assassin's Creed III game next year (so, think Assassin's Creed III: Brotherhood, etc).
The latter is almost to be expected, since Assassin's Creed II got two sequels, but co-op would be a welcome and long-overdue addition to the series.