I would love an mmo of Mass Effect. In fact, there's a section in the ME2 credits that lists an mmo team, if I'm not mistaken. With the amount of money that mmo's can make, and the sheer size of the mmo community, it would be ludicrous for any company not to try and tap into that demographic.
I know some people are anti-mmo because you cannot convey a decent single player experience (i.e. dialogue, choice, etc.) and I can tentatively agree there. I've played a number of mmo's, but only WoW kept me for longer than the trial period. And for all of those games, I would have to agree with there being that lack of a single player experience. That said, I have not played the Old Republic, and everything I'm seeing in regards to that game seems to be that they're trying to bring choice, story, and dialogue into an mmo.
So depending on how the Old Republic is going to turn out.. I'm down for an mmo. I think what the majority of gamers want, is the ability to play with others cooperatively, and show off their own character, while also having that epic story line. The mmo genre is the best candidate to accomplish that. In a limited/small scale, borderlands also did a good job. Remains to be seen if BW can accomplish that or not. I'm hoping for yes, but we'll see.