This game desparately needs more zombie models and more variation in zombies' abilities. Even a beginner notices after an hour of gameplay that there are the same zombies all over the place and they can't do anything but getting close and swing after you. They don't have to be completely new models from scratch with costly artist work required. Variation in colors, maybe just a variation of faces on the same body, add some scars here and there for variation, different hair, let some zombies spawn with only one arm etc.
Exactly but actually they are on a pretty good way. There are tons of options to improve the diversity of zombies and zombietypes without crazy stuff outside any zombielore.
Spiderzombie: small, very agil, can jump and climb, fits perfect.
fat zombie: much HP, exploding is nice (seen in dozeens of Z-games, they just explode)
burned zombie: can ignite you
fat mama zombie: tons of hp
crawler: crawling!
demolisher: no zombiebreaking lore, he has just a bomb on.
screamer: very nice!
army zombie: headshot resistance.
football zombie: faster or perma running
There are allready a lot of different styles based on the type of zombie.
There couldt be easy much more:
stripper zombie: long nails do scratch/bleed dmg
muscle zombie: very strong, has a short range charge attack.
bulldozer zombie: fat and fast. accelerate and wanna crash into you.
half mutated zombie, infection not completed: can use weapons, has creepy voicelines because he can still barely talk.
the doctor or scientist: still as zombie more clever than the rest, he can open doors and acts smarter in any way.
police zombie: bulletproof jacket, aim to other bodyparts.
nurse zombie: care for infections
feral wight: running at night (see below)
This are just the ones without crazy stuff. Spitting zombies, poison aura or any other superpowers are possible too.
When there are all different zombies, different strenght and weaks, different movespeeds, behaviour and special abillitys you can finally reduce the green horde to a minimum and get rid of this "running at night" feature because its all coded on the zombies. Mayby some spawn only night, hide at day.
Some mods out there made allready epic stuff with the new debuffs.