With the end of Shepard's story, multiplayer being present in ME3, assets for a multiplayer game mysteriously having been pulled for something else, the infrastructure for an MMO already in place, and established playable races (along with their respective classes) and a Universe for them to live in, I began to ponder the possibility of this.
I can't help but wonder if--a bit down the road--Bioware are going to have a surprise announcement regarding this, and it isn't that hard to deduce, really. They've changed the Universe, and thus decentralised things from the council. Thus - homeworld starting locations make a lot more sense. I would almost wager to say that it's going to be set a century after the end of ME3 - and experimental new jumpgates or jumpships are now being deployed to carry people around in the absence of mass relays.
Mass relays having become a memory, a myth, much like the legendary Commander Shepard, who's going to be an important theme throughout it. I suspect that the upcoming ending segments may indeed allude to this possibility.
I wouldn't mind an MMO with the sort of combat and dialogue that Mass Effect has. It'd be a huge step up from The Old Republic, that's for sure. Furthermore, they have discussed a potential MMO, so perhaps a reveal isn't that far away - they do have a lot of the resources already created, and we know that Bioware does tend to reuse stuff.
Consider that in many ways they don't have to redesign races, technology, or even architecture or culture. A very valid reason could be used for these having been put on hold - it was imperative for everyone, as largely space-faring peoples, to concentrate on new travel technologies, and to improve on what they already had. This took more precedence than other things, so it became a... well, a space race, if you know what I mean.
So... I'm putting money down on a Mass Effect MMO announcement being just around the corner, and if I'm right - I called it.
Also consider the possibility that after the flop that SW:TOR has been (financially, talking about profit, not revenue), they may actually be considering a buy to play model, similar to Guild Wars 2. I base this on the packs available in Mass Effect 3 multiplayer.
It does look like they're testing the waters, doesn't it?
As a personal aside (though this is unlikely), I'd actually love to see them adopt a Guild Wars 1 style approach with their likely inevitable Mass Effect MMO-ish affair, where you have planets and bases as massive player hubs, but you dispatch as squads into missions. So in your mission instance, it's only you, your friends, and friendly/hostile AI.
Perhaps have some areas to dispatch to for war-style scenarios too involving a lot of players, but I think that a heavily instanced approach like that would work really well. At least for this particular Universe and game style. But this is, perhaps, wishful thinking.