1-Rare food type items (cans of food, pepsi's, cereal, rice, etc...) to enhance survival, give a more thrilling experience and high reward when finding said items. Give more reason to hunt and cook.
2-Flies animation and sounds on decaying human/animal/infected bodys to give an immersive/apocalyptic feel and give the player an idea of how long the body has been there for. (10min+ or 20min+ until fly anim and sound, then maybe decay visual process of bodys then at 40 min mark despawn)
3-A reason to use a lightsource at night, maybe make inside of houses darker than outside to give the player a fun thrilling survival feel of looting in a house at night with a flare/torch/chemlight to find items that hide in the shadows, or items under beds etc.
These are just some ideas I would love to see ingame...