Have you ever played co-op in Tom Clancy Splinter Cell Chaos Theory? or Far Cry 3? or Tranformers WFC? Those are really great examples of how co-op can be implemented in CP77, or at least they are examples of how I'd like to play co-op in CP77.
I think it should be possible, during co-op, for players to talk to each other and decide which objective they're each going to handle for the mission; so when the mission starts, the players can go off on their separate objectives, meanwhile maintaining communication with each other (all of this is pretty standard in games which do this).
So at the very least, co-op options should allow you to add players (by importing their characters) to your single-player storyline missions. I would also like if players are allowed to make decisions together which affect the single player storyline; of course, anyone who doesn't want to do this, can just tell the other guys that they don't want to do this.
It would also be interesting if players, when added to someone's single player storyline, can affect the story and gameplay for the host via antagonistic actions; in this scenario, it should be possible for the guests to interact with game world NPCs and story aspects in meaningful ways. But of course, both friendly and not-so-friendly actions should be allowed in such a mode, and the host should ultimately have the capability to chuck out players or deny them the option to act in unfriendly ways.