Had a few ideas too.
There should be some variants to the wave based survival mode script:
Timed Waves: A new wave begins after the given time runs out. (A normal wave could last a minute or two)
Limited Waves: Waves of zombies increase until a set wave. A new wave begins 10 seconds after all the zombies in the previous wave are killed. (Wave 10, for example)
Endless Waves: Waves of zombies without end. Again, a new wave begins 10 seconds after all the zombies in the previous wave are killed.
The gamemode could also have a client based weapon and perk store or something