Call of Duty: World at War has a co-op campaign with up to four players. The reason Black Ops and Back Ops II do not have a co-op campaign is because the stories were designed around one player, which allowed for a more compelling storyline.
You mean like what Respawn Entertainment are doing with Titanfall? Where there is no campaign, but the multiplayer mixes with some sort of a single player experience. Example: A cutscene tells you where you are why your at that certain location, then you do the objective through the multiplayer experience. Yes that would be a cool idea, but I like the campaigns like they are at the moment.