When it comes to zombie spawning, hiding and so on. I think that it would be a neat idea to alter stealth mechanics and so. Here is a neat idea to make it "balanced".
- Add probability based time checks and bushes/soft sight blocker objects. Basically if a player is standing in a bush, instead of relying on zombies sight only and so. Give the player a random check every 30-60 seconds. If the random check succeeds they are hidden for the next interval. The interval would be based on sneaking skill as well as the chance too. Meanwhile NOT affecting visibility without soft sight blockers, such as bushes. Also anytime a zombie looks at the direction of a player who succeeded a check like this they somewhat steer away from them, meanwhile for a failed check they remain on base AI mode.
- External sounds should matter more. When a sound gets emitted make it so that zombies are likelier to notice it. Instead of making it a "zombie hearing range" it should be a "sound contest". If the source of sound is in the zombie's hearing range AND the sound range of the sound is also in the same range, they heard it.
- Disabled zombies and less aggression. Basically zombies with only 1 eye, no ears or no vocal coords. Zombies without eyes cannot see. Zombies with 1 eye can see, but lose track fast. Zombies without ears cannot hear, zombies with 1 ear can only determine direction. Zombies without vocal cords cannot make sounds from grunts, etc. As for the less aggression, make it so that zombies DO hit doors, BUT they occasionally stop and forget every player presence. kinda like a "gave up on pursuit".
- Slightly more intelligent zombies. You need to lock the doors or they can open it. But only like 1% of zombies or so.
- noise and visual cancellations. (this is a bigger one) basically a player activity heat map tied to light and sound. If certain sounds and visuals are played BUT without player activity, those would normalize for AIs and be subtracted when considering all.