I suggested elsewhere that loneliness could be a serious contender for filling up the end-game, so let’s chew it over. For starters, “loneliness” is kind of a catchall for the psychological effects of the Cataclysm. Grief for loved ones, purposelessness, and, of course, craving for human contact are all bundled in here.
Mechanically, what I had in mind was Cata secretly tracking the extent of the survivor’s isolation, winding down from 100 (healthy social life) to 0 (utter hopelessness and despair). Each day it ticks down, imposing morale penalties and threatening the character with mental illness. There would be various methods for keeping the survivor semi-sane, most prominently reading those “Just for fun” books, keeping pets, talking to NPCs on the radio, and establishing your own “tribe”. Of these, only the books and radio are easily implemented, although perhaps getting the Cat Food to summon you a housecat “item” to carry around and keep you company (it’s constantly on and needs to be “reloaded” with food from time to time) wouldn’t require too much novel coding.
Obviously, these methods would have different levels of effectiveness, but more importantly they’d be limited in how far from 0 they can take you. Reading can take the edge off, but it would be limited to pushing the character’s loneliness off the bottom (can’t go over, say, 30). Being a cat-lady is pretty insane in itself, so maybe taking care of kitty only gets you up to 50. And radio contact still isn’t the real thing, so that’s a ceiling of 80.
The great merit of this scheme is that it doesn’t much change the opening, so new players aren’t going to get clobbered. They might see a “You miss your Mom” message pop up, but if they’re struggling to beat Day 5, loneliness isn’t going to do them in. For veterans, the trade-offs between keeping a library but being mentally ill, having a fussy feline but being a bit gaga, or doing quests for your radio pals so that they’ll talk to you, adds more difficulty, not to mention realism, to the late game.