I was watching the movie called The Fog and I had a idea very similar to this. Basically the idea goes every round that goes by a fog will start to shroud the map making it slowly harder to see for humans. Wave 1 the fog will only be in Zspawn and at wave 6 the fog is ever where but buildings. This can help reinforce the idea of teamwork as groups of survivors that are braving it in the fog have to stick together and move slowly. Of course it would harm other aspects of the game like sniper rifles. But the fog would only be a small chance in the game and some other tools to help as well like spot lights and a trinket called thermal IR goggles which almost removes the fog from view. It'd make having a turret more valuable as turrets won't be affected by the fog. It forces humans to play more carefully as zombies could be hiding from any angle. It'd make walls less impenetrable and also make it harder for Hteam to communicate with each about what boss is leaving or not. Overall I think it'd a good way to spice up gameplay and make it more intersecting as the same formula of cade and defend can get a bit boring.