You could make the zombies look a bit more realistic if you are going with a realistic theme. I recommend learning how to model and code these weapons. If you want some ideas I will list them below
Ideas
Add secrets to the map, e.g Easter eggs, secret weapons.
Add seasonal events that could grant some permanent tools.
Have a looting system, maybe zombies can drop food or maybe tools that could be useful later on.
Add variations of zombies that do different things, e.g a Moon Zombie that has more health at night.
These are just a few examples of which may have an impact on the game. Keep up the good work!