Diseases (if you stay in cold place for some time you will become sick)
Infections (the bleeding wound could become infected if you don't bandage it)
Cooking (making 'higher tier' food from basic ingredients and making alcohol)
Improvised electronics (volta's battery etc)
Psyche of NPC's (they could go mad if they're hungry, scared, thirsty, wounded etc)
Gasbottles (for cooking, killing zombies...)
Skills (increasing when doing actions that use them: if you often repair weapon your skill in using/conservation of weapon will rise)
Somethang controversial things (dead kids, zombie kids, zombie seniors, heavily wounded zombies, cause it's APOCALYPSE and these things should be in really apocalyptic game!)

Crazy Serious Sam-like slaughter?