The Assassin's Creed series has dabbled in cooperative concepts, like calling in AI reinforcements in Brotherhood, or the "Wolf Pack" multiplayer mode in Assassin's Creed 3. But Ubisoft was actually considering the mechanic from the very first game, and it was subsequently cut.
"Before we knew about the Desmond story and Animus link, we had a huge co-op component in there," AC3 mission director Philippe Bergeron told Official Xbox Magazine. "But it just became too hard to do: the engine couldn't support it, and then the metaphor we had above it didn't support it. Co-op was one of those big things at the beginning that just didn't make sense in the end."
He said aside from the engine constraints, the team couldn't find a way around the story problems. "There was no way to reconcile having multiplayer or co-op in an ancestor's memories. Your ancestor lived his life in a certain way, so assuming you had branching storylines, it creates a paradox. It didn't fit."
A recent survey asked if players would be interested in playing Assassin's Creed with a friend, renewing speculation that the series will have some kind of expanded co-op element in its future.