I'd love to see a map similar to Hard Rain in Left 4 Dead 2. It's a campaign that takes place during a flash flood, and your vision is obscured by the storm. Every once in a while, the storm really kicks up, obscuring your vision even further to a level similar to the fog in Tranzit. During this, the storm drowns out the player's microphones. There'd also be flooded areas where you'd run slower.