I think this could potentially be a nice addition to co-op to keep things fresh and harder due to the need for good target acquisition. This would add to the immersion, difficulty and chaos that comes with co-op, almost making it more like a campaign.
The Idea: Having random civilians NPC's around the map doing various actions to add to immersion and difficulty. They would only need a very basic AI to complete these actions.
-Cowering in the corner of a room
-Running out of a building to avoid the firefight
-Running away from combat
-Staring out of windows in non combat areas or cleared areas
-Randomly moving from point A to B then cowering or despawning
-Standing around in non hostile areas, but running if combat comes close
Potential Punishments for killing too many civilians (not all of them at the same time, just suggestion as to a mechanic for punishment):
Too Many Civilians Killed: You simply get auto-killed and you respawn at the next cap
Court Martial: After the objective is capped you despawn and sit out until next cap
PTSD: After killing a civilian, you simply get locked out of shooting for a limited amount of time after about 5 seconds (just like when you enter enemy territory and are told to turn back)
Disarmed: You are relieved of all of your gear, and you have to acquire your own provisions
Captured: You get transported to a room or area where you can only look around and have to wait until your teammates break you out
Pushback: Killing civilians spawns more insurgent in the area
New Recruits: Killing civilians causes defend phases to happen more often and scale insurgent numbers higher the more civilians were killed
Let me know what you think! Thanks for reading.