I'd be interested in a First Contact War prequel, if I believed in prequels.
But I really don't. Damm you Lucas!
Given the choice, I'd prefer something set much later, after the Quarians re-fortify and re-establish, maybe get some help from the Geth, the Council rebuilds it's military and steps up it's command structure, Humanity is maybe finally established as a significant galactic power, instead of constantly huffing to make even.
And in this setting, I'd choose Human, or maybe Turian. Certainly voiced.
As for those who think that the ME universe is too unified for different origins, you forget the Terminus Systems, maybe there's feudal Batarian remnants and other races trying to move in on former Batarian space. There's always the independent planets scattered around, like Illium. Or the HUGE expanses of the galaxy that the Council keeps unexplored, maybe there's a New Common Enemy. Humanity wasn't around during the Rachni Wars, it would be interesting to see how another Relay War would turn out with an established galactic human power, and the fallout regarding the genophage. Maybe the Krogan could be a race on the fence, still remembering what happened last time they helped the Council, but also remembering that their cure came from Humans, Salarians, and Turians throwing out their old doctrines and working together, meaning that Bioware could have a second chance to do the whole 'gather alliances and fight a REAL galactic war' thing again, but maybe not horribly wrong this time.