Before there was Desmond and the Animus, Assassin's Creed actually had a cooperative mode in the works. Assassin's Creed III missions director Philippe Bergeron revealed as much, but the co-op mode was ultimately trashed when developers realized it probably couldn't be done -- at the time, anyway.
"Before we knew about the Desmond story and Animus link, we had a huge co-op component in there.
"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."
The co-op mode that never was became one of those things that just sounded better going in, but ended up being less feasible in terms of the bigger picture. Given the direction Ubisoft took the series in, co-op just didn't fit in very well. Bergeron goes on to comment:
"For us it was really part of the single player experience, to have in-and-out co-op, and in the end we never thought it made sense in the storyline that we had for the Animus.
"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."
Obviously, Assassin's Creed did get its multiplayer component, for better or worse. Introduced in Assassin's Creed: Brotherhood, the series' distinctive style added a unique addition to online competitive multiplayer. In Assassin's Creed III, Ubisoft did offer a variant that had players teaming up rather than facing off.