Let's indulge in imagination
Livingdead: Inspiration by Alex Mercer from Prototype and Vincent Valentine from FFVII. They're mutant zombie with humanity, different from normal zombies, they have the same appearance as living sims. If they were occults before dead, they still have those occult powers. Same as ghosts, they can't die but can resurreced, they also can't bitten by zombie.
Livingdeads haven't bladder need, hygiene are replaced by freshness. They don't get much freshness from normal cleaning, they require formalin. Once a livingdead's freshness bar is empty, they'll get a strong uncomfortable mood "Rotten", it will make other non-livingdeads get a strong uncomfortable mood "Rotten Odor" and make them vomit (other livingdeads same as normal). They won't lose freshness faster while take exercise; but dumpster diving will.
Livingdead can't get any hunger need from normal meal, they require eating raw meat, brain fruit meals, Brain Freeze a la Mode, and bite other living sims. Same as vampires, livingdeads have three options for bitten living sims; sims that bitten by livingdeads won't turn into livingdead, but will get different degree uncomfortable mood. Once a livingdead's hunger bar is empty or very angry, they'll bitten other living sims uncontrollably.
If a livingdead is sleeping, they can't be woken up, they're completely motionless while sleeping. They're immune to temperature and won't breathe white when cold. Non-livingdead sims will get a tense+1 mood "Lifeless body" when have a body touch with a livingdead, same as aliens, they can be found as a livingdead.
Livingdeads have specific interactions with other sims; they can "Talk about Grim Reaper", "Enthuse the second life", "Enthuse undead", "Complain about rot", "Imply rotting faster" towards other livingdeads; "Talk about Grim Reaper", "Show off body", "Enthuse undead" , " Talk about body or phantom", "Argue body or phantom" towards ghosts; "Talk about livingdead life", "Mock dead" towards living sims.
Livingdeads have specific bed: Anticorrosive coffin. Only livingdeads can sleeping in anticorrosive coffin, their freshness won't lose when sleeping in anticorrosive coffin.
