I would vigorously, enthusiastically, and joyously support the development of a ME MMO
IF
AND ONLY IF
(Now read that again to make sure you get it.)
it can be done decently, without straying too far from the ME class-based shooter-type gameplay, with a strong (Though not necessarily nonlinear) story and without the genre cliche of two opposed factions that never interact apart from shooting each other.
Why: As Casey said, the ME universe is impossibly vast, and in the end we are saving the galaxy so we, and everyone else that lives, can live in it, and a well done MMO, I think, offers that potential. MMOs also in some sense free writers from having to have a huge galaxy-threatening story. Casey specifically said they built major world hubs intentionally so they could never run out of stories to tell, and they are all, I'm sure, worthy stories--It's just not the fate of the galaxy at risk, and that's ok in an MMO where it isn't quite so much in other kinds of games.
The gameplay is already divided into classes, major races are already known, as are potential bonuses, and lore is established. Equipment would probably take a little more from ME1 than 2, but again, MMOs are more accustomed to that interface and setup.
If this MMO could be made, and could be made well, without compromise for the sake of a release date or money, I would definitely fork over a monthly subscription. And that's coming from someone who, in every other case known to man, has refused to do so, and proclaimed that payment model to be dead.
If, on the other hand, any aspect of the game was allowed to be sacrificed, even slightly, I would ardently fight the development of the game.
Basically, give the ME team a blank check and no time limit to do it, or don't do it at all. This is one of those things that would be beautiful if done precisely how it should be, but ugly if it's even slightly off.
I let Bioware make that call. I have faith in the ME team's ability to write, design, and listen to sense. I believe they could do it. But it would not be a quick or easy thing.
Barring an MMO, the First Contact War would certainly be interesting. It'd also be cool to have a game where you could pick your race--again, that's a big draw for an MMO, but if it could somehow be fitted into another story in the ME universe, it'd definitely be cool, particularly if that one had multiplayer--Though, again, that is still along the slow creep to an MMO.
Maybe a sort of Fable-ish type game where you start out as nobody bounty hunter and eventually build up to making your own mercenary band or somesuch? Might be interesting.
There's no shortages of places to go, honestly. My biggest interest will be in what other directions they take gameplay--Be it a more traditional shooter in, say a First Contact game, or maybe something entirely different. Mass Effect puzzle game?
Now you're thinking with eezo.
... Yeah, I'm gonna go to bed now.
-Tolan *Is shot for making an ME/Portal reference.*