Just a little idea. Basically, little actions that certain types of creature take that would make sense in-universe for the creature to do. They wouldn’t have too much of a gameplay effect, but they would have an impact on some parts of the game. The most minor actions would be essentially “[NAME] does [THING]”. Zombies would moan/howl, NPCs would cough/talk to themselves/yawn, wolves would howl/growl, etc. These could let you know that something’s in the area, though there should be a “stealth” mode for certain creatures where they don’t make any noise. Actions that would have more effect would be eating and sleeping. Zombies would find human/animal corpses and attempt to eat them, animals would eat vegetables/corpses (excluding zombies) depending on if they’re herbivores or carnivores, NPCs would butcher creatures to get meat, raid stuff from fridges, or just eat from their own personal stash. NPCs and animals would also return to a “home base” (Basically, whatever’s considered to be the safest place) at times where going to sleep would make sense, like nighttime, and go to sleep on whatever would be the most comfortable. They would wake up at loud noises and investigate.
Might be a little complex, but it could provide nice atmosphere. It shouldn’t be too overdone, though.