I don't think this is a good idea at all...I want a single player RPG experience, and I want the devs to focus their time and efforts on perfecting that experience rather than be distracted by other modes of playing, or having to adjust the whole game to accommodate that mode. There's no need for it really, that's the kind of thing publishers like EA mandate to their devs because they feel the need to hedge bets and appeal to the widest possible market..IE Mass Effect 3.
That's the perfect example of spreading out resources too thinly..ME2 was awesome to me, because it was well-rounded in both main story and side-quests/exploration. I wanted to pick up and play that game all the time. But in ME3 they had to throw in multiplayer...and look at what happened to the main game....dozens of useless, lazy fetch quests that were a time-consuming chore to complete, zero fun, and only required Bioware to record a few lines of dialog to generate. That stuff totally took me out of the game and my motivation to play it (I bought it on release and am just now about to complete it).
Don't get me wrong, I love co-op gameplay, but in a game designed around co-op, not in my hardcore RPG. I want the focus of their limited resources here to be placed into game detail and depth...like everyone else I don't want to see a very good game, I want to see a genre-defining one, one that sets a new industry standard.