I think having some NPC backstory missions where you can play Co-op would be great. Main story doesn't really suit co-op. Unlike RPGs like Icewindale and Neverwinter Nights where Co-op is brilliant.
But then again Co-op is all about playing your favourite game with someone. So even a full campaign Co-op would get my approval, because I'd love to play a game like this with someone. However they'd need to design some fire fights that require a more tactical approach so that you can discuss stuff with your buddy abotu how to handle the battle. I just found with ME2 that I could more or less camp, clear out, advance, camp clear out advance for most of the missions. I rarely had to actually manage the placement of the other squad members.
I'd prefer to see an increase in mission squad members from 3 to 5 and, like in Dragon Age, allow the player to take direct control of squad members, so the player can properly micro-manage battles. In those sorts of set ups co-op gameplay is lots of fun.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
