Well seeing how halo did a prequel along with a nameless soldier game, there is always that option. They are doing another supposed trilogy with halo 4 and beyond but at this point they should've just let it die. Gears loves ripping off halo's ideas and is doing similar stuff with "Judgement".
They should let ME end with whatever scraps of dignity it has left, and they can't really do a prequel since existing decisions and save files would have no effect, and no one is gonna play a prequel then replay the main trilogy.
A somewhat-canon game that could be cool is having all the fleets and races cut off from their home planets and stuck on Earth, and then have a bitter war break out over resources. The game would have no space travel due to the premise, and previous decisions would play in and any unknown betrayals like genophage sabotage could be revealed, but the main objective is to rebuild a relay system and try to get everyone home.
But if they don't just bite the bullet like they should and take a ODST/Reach path we may very well be seeing "Mass Effect: Nameless Alliance Jackass" hitting shelves in 2013.