I'm getting more than a little sick of prequels in the entertainment industry as a whole. No way no how would I buy a Mass Effect one.
I had the same initial reaction to Halo 3: ODST until I actually played through a couple of levels; wasn't quite the bastardized "Because we can always make more money" prequel I thought it would be, and by the end it was actually pretty entertaining.
And the Mass Effect universe is FAR better developed than Halo's. It's practically soaking with sidestory material; the occupation of Shanxi, The Skyllian Blitz, the Battle of Torfan, the sacking of Mindoir, the Adventures of John Grissom, the founding of the Blue suns, the Collectors invasions of Feris Fields... any single one of these is actually fertile ground for a fully developed RPG, provided you're willing to expand character and story development beyond the initial premise. The battle of Shanxi would probably be the easiest, though, since the story is already mapped out: it begins with the turian invasion of the planet and ends with the intervention of the Citadel. That process between those two points was always sort of glossed over in the Codex... but then, EVERYTHING in the Codex is written that way, even shit you did yourself and you know good and damn well it wasn't nearly that simple.
For all we know, Shanxi involved running land battles on three continents, the theft of a Turian frigate and subsequent ramming of their flagship, fleet actions and asteroid bombardments in twelve neighboring star systems and a guerilla-style infiltration and sabotage of a Turian shipyard. Throw in some Batarian intrigue, a "Priority: Search the Galaxy for Allies" mission, and maybe even a before-he-was-famous cameo by the Illusive Man, and you've got yourself a game.