I say, increase the number of spawned zombies in the beginning but decrease the respawn rate (a lot) or the number of respawned zombies after the first spawn, that will increase the need of think and generate a tactic to enter a citie or small town to get rid of most or all zombies for enough time to be gratificating and dont feel that is a complete waste of time. That or make the ppl killed by Zeds become a zeds with zed ablities... that would be scary... groups of zombies that actually think... that would rlly push ppl to team up