Assuming the game would maintain its incredible story telling methods and mechanics.
Personally,
an mmo would be cool because the universe is so damn cool and the races are awesome but quite frankly it would have to be so out there and different to make it in the mmo market. I would want it to be a 3rd person shooter mmo.
RTS, mehhhhh.
FPS, They were originally gonna have the Me3 multiplayer be fps but IDK. Once again this would have to be radical and stand out from other fps's or else there would little reason for the change in genre.
Honestly, if they pulled of a Cinematic game, like heavy rain, and they did it well, then it would be very interesting. Firstly because, lets be honest, the current combat is still mediocre so it wouldnt be missing that much, and secondly it would ramp up the intensity and story telling. Of course, they would need to have a new protagonist or story because shepard is a shooter first. This would be interesting
Also tell me if you have any other ideas!