A simple sugestion, Entertainment.
Allright, I've been thinking. The world's pretty depressing in this game. Heck, even in real life, people need escapism. I'm going to sugest that if one isn't "entertained", like from maybe games on a smartphone, Drugs and other things (Maybe zom zoms?) he'll get depressed. (Which could also be a trait, similar to motabolism which makes those things less effective)
To stave off depression, One could recreationally use some drugs, like painkillers, Drink booze (Maybe even able to make a makeshift still, to turn berries into wine?), Or just fuck around on a electronic device, like a laptop. Could even implement handheld consoles. Ofcorse, to power them on long trips away from the DMC, One could get lucky and find an extremly rare solar panel to charge them with...
Sorry if the sugestion's too sims-y, I was recently replaying fallout 3 wander's edition.