I would love a Mass Effect MMO, unfortunately since the current model for MMOs is total garbage and any MMOs that would be released would be garbage then I would have to pass on the idea for now. (I did enjoy the thought behind the SWTOR interactive cutscenes and character stories though, was a nice shot at something resembling a roleplaying game albeit lackluster.
If at some point in the future a company can create a massive game spanning the entire mass effect universe, implement a good overarching story, and give it solid mass effect shooter gameplay then I would be open to the idea. Although I think the game would have to be less of an MMO and more like something the original Guild Wars was. Just an Online game where you could gather in social hubs like the citadel, or other cities, but traveling outside of a city was always instanced to you and your party.
The current idea of being able to see other players wandering the world in good in theory but in practice it completely takes the player out of any sort of roleplaying experience. LIke.. "Hey look at that krogen over there jumping in circles and killing hundreds of Varrens to collect hides, I think I will join them" thats just stupid gameplay. Everytime I leave town I would want it to be into a mission, like one of the ME2 squad stories. Scripted and linear.
In the end, YES I want one but NO it couldn't be done.