^The trouble with that is that by all accounts, there's going to be a very wide variety of possible outcomes for ME3, making any sequel when you can import your old saves somewhat untenable. They'd be forced to choose a single 'canon' storyline and move from there, which would be a shame considering how personal the Shepard trilogy can be to the player.
I agree that the First Contact War isn't enough to sustain another trilogy of RPGs, but I think it's plenty for maybe just the one or a straight-up FPS, *if* they were so inclined.
In terms of prequels though, it's hardly the only option, even if the presence of humans is a pre-requisite. There's also the Skillian Blitz and the subsequent counter attack against the Batarians that can be part of the 'War Hero' & 'Ruthless' backgrounds.