First things first, I was totally against multiplayer in ME3. The series has been a single player experience and should remain one (for the most part).
What I wouldn't mind as multiplayer are 2 things:
1. Coop horde mode style where you replay various big battles going on in ME3 as eg Turian or Krogan ground teams in coop. Could add a bit to the experience and see things from a different point of view.
2. The option to have a second player join in the campaign and control a squad member. This is mostly because after I do my playthrough, I have like 3 friends who all starting playing ME because of me and I watched them playing it (most of the time). So I would like to know what things and decisions happen in their playthrough. -> Wouldn't mit picking up the controller and join in on the fun a bit
But anyway, let's wait and see. News should be coming in soon.