The Infected and their Buildings.

 So this idea came into my head, If there are infected everywhere, and they are more dangerous at night/darkness, there should be random buildings/houses in the game so we can.

Earn exp from killing/ scavenging

If we clear the entire building, and unlock most of the doors (Kill counter of zombies left would be necessary) we can take it over.
Imagine a Encampment, inside the chainlink fences of a former military base! with make shift Barricades and fence patches to stop the infected from coming in.

or a warehouse, with all of the windows boarded up on the first floor, and plants growing on the 2nd.

Also, tunnels (drainage sewer lines) where infected can spawn from. (If bombs ever introduced, Infect cannot spawn for 24 hours, because they are digging out)