Several sources suggest Mass Effect 3 will feature a four-player co-op mode. Which will be standalone, online enabled with competitive focus so the multiplayer aspect of the game doesn’t compromise the singleplayer campaign.
Mass Effect 3’s multiplayer will be a Horde-style mode, which will challenge players against waves of enemies.
Eurogamer claims, Bioware Montreal was working on the Mass Effect multiplayer experience, either as a standalone game or component of Mass Effect 3.
We don’t know how accurate these claims are, but it will definitely enhance the overall experience of the game. Hopefully they won’t screw up anything.