Well this is not a problem on PC but big problem on PS4 and Xbox One. We have single player and multiplayer games but we don´t have any co-op games.
I think Valve should make Left 4 Dead 3 quickly and release it on PC, PS4 and Xbox One. (Yes, release before HL3).
People need some good CO-OP games and right now we don´t have any. I miss some good old co-op games :(
I would like to see Portal 3 in future but not now. It´s great game but not good as Left 4 Dead 2 co-op.