I think Multiplayer could be a cool feature, if done right. I thought mulitplayer would have been a nice touch in Mass Effect 2 after playing it a few times. Since Mass Effect is a story driven game, I feel that's how the multiplayer should be. Take a look at Bioshock for example. It had an okay storyline, and a cool concept for it's multiplayer. Though I felt the multiplayer wasn't very good.
I don't necessarily mean competitive multiplayer, more cooperative. I think there should be a co-op campaign. You would be able to invite two of your friends to join you, to stick with the three person squad. The party leader would be Shepard, obviously. Rather than Shepard choose which squadmates he/she brings, the other two players would choose who they would be. The same would go for the weapons they wield, and how they're upgraded.
I think this would constantly keep the game fresh, and would certainly be fun to try on Insanity.