It comes as no surprise that Mass Effect 3 was not going to have multiplayer, as the most recent GameInformer has confirmed. The premise of the franchise thus far has been the story of your Shepard, so adding a multiplayer experience on top of that would seem to undercut that goal, as well as distract from other matters on which they could be focusing (does anyone like slapped together multiplayer that's not built with such in mind?).
That does not mean having a story that doesn't focus on one person in the future is out of the question. According to VG247, cofounder of BioWare Casey Hudson had the following to say:
We've been trying to think of a way that makes sense for people to experience Mass Effect with their friends. We haven't yet come up with a way to do that, so we don't have anything to announce at this time. But, obviously, multiplayer is something we want to do more of in the future as a company.
A lot of people say that they want to see an MMO, I think that kind of makes sense for this universe. Part of what you're trying to do is save the universe so you can live in it. That's part of the promise, I think, for any great IP. It has to be a world worth saving... I think Mass Effect has that quality to it. If you get rid of the Reapers and win that, wouldn't it be amazing to just live on the Citadel or just take a ship to Omega? That makes sense.
Of course, multiplayer for BioWare isn't wholly new, as I often used the function in Baldur's Gate to completely build my own party to try things. Then again, I can't speak to the experience when playing with others, so I suppose Star Wars: The Old Republic will be a strong indicator of what BioWare can do in that direction.
The larger question for me is wondering which worlds I would like to experience as an MMO, and whether or not that means much of anything. I was always of the mind that I would have preferred an MMO based off Blizzard's Diablo franchise, rather than Warcraft, but it's difficult to argue Blizzard didn't change the MMO landscape and do well with that particular name. In this case, Mass Effect does seem like the franchise to focus on rather than Dragon Age, as the former seems to be concluding soon, but there's still a lot to explore in the changing world of the latter.
Are there any other games/series whose worlds have entranced you enough to want to be able to explore it as an MMO, or seem like a particularly apt fit?