Gamereactor interviewed Jean-Sebestien Decant, the lead game designer on Assassin's Creed IV: Black Flag, and they talked about the possible future of the Assassin's Creed series. Decant explained that Black Flag wasn't truly a next-gen game, and that the future games in the series will really be next-gen. For reals this time.
He described next-gen as "going toward a gaming place where the open world will be shared with other players." If that sounds a little bit like Watch Dogs, that's because Decant admits that Watch Dogs "is kind of showing us the path for the future."
Personally, I hate the idea of other people walking around in my single-player game. If I want to play multiplayer, I'll go play multiplayer. If I want to play alone, just let me play alone.