Until I have a better name for this level it will be referred to VS_L4D2_1.
This map will be completely map script controlled, I'm sure i could do this with the original GoldSource engine so it shouldn't be too hard to implement in l4d2. Remember this is the conception phase so if something doesn't seem right please speak up.
OK the basic flow layout

Z1 and Z2. Are the area zombie will be aloud to pool up during each wave of combat. If the director can't be completely controlled by the map based entities then a system of doors can be implemented [more research needed]
Z Kill Zone: Player's will be responsible to kill the zombies that are running down the Z killzone towards No Mans Land.
No Mans Land: If a zombie reaches the no mans land the map will first detect that this has happened and trigger an counter inside the map. Then after its been counted as getting through the zombie will be destroyed by a environmental danger [Much fun over which type of damage to deal]
Base: The players will have to live inside of the base. If the player attempts to leave the base into no mans land this will result in immediate termination of there life for the round. There will be a warning.
Counter Function: The counter will count up to a set amount of zombies that slipped by. When the counter hit a set amount the round is over all players are then terminated.
How many waves or how long can the team last will be the point of this map. I hated how the zombies had a great advantage over humans in an open environment. In this scenario the environment is controlled. Just don't let any slip by.