About smell, would it be interesting to give players a basic sense of smell?
Certain items, actions or entities could emit odors, such as certain food items, explosions, animal dung, turned, etc. This allows for a slightly more realistic way of perceiving things along with hearing and vision. Smells would be indicated by a small notification somewhere on the screen, with a circle that fills up the closer the player gets to the source.
(Small example)
This allows mostly for tracking players, animals and items, and even maybe as an early warning for special zombies (e.g. being near an acid spitting zombie could say “The air is acidic”). It’s a new gameplay element that players will rely upon, which means you could trick players: a zombie/mutant could emit a sweet smell, showing the same notification as if the player was near fruit (The air is sweet), for example.
As the old adage says, everything leaves a trace.