This is actually a serious idea, but undoubtedly also horrible:
Eating is a big part of most roguelikes, and Cataclysm makes sleeping a big thing. The next logical biological function is…pooping and peeing.
You could have a bladder indicator, telling you had badly you need to pee/poop. Particularly dangerous encounters and drugs could make you have to crap immediately or piss yourself, and bad food could give you uncontrollable diarrhea. Pee could be used in emergencies instead of water (maybe boil it to make it safer), and poop could give off powerful scent clouds to attract zombies, and be usable as a thrown weapon or fertilizer for farming.