My idea is a fairly simple one, really.
Undeads get inate bonuses to damage reduction (piercing, slashing, death magic, cold magic), some vulnerabilities (fire and blunt weapons), immunities to sleep, stun, mind control (basically anything that affects the mind), poison, bleeding, the obvious stuff. They get a substantial bonus to HPs.
On the other side, they can't heal unless they level up. They might stumble upon a powerful artefact now and then, or get a very rare opportunity to suck the life out of something, find a fountain of undeath, I'm just shooting vague concepts. The idea behind that would be punctual and rare findings that might restore some health for the savvy explorer.
The real trick about keeping the Undead alive would reside in the player's ability to manage a big health pool that just keeps going down. That would be tactical choices on the battlefield, knowing when to protect the character or have her fall back, adopt defensive strategies to spare some valuable health, as well as outside combat, knowing when to pick a fight, chosing to avoid conflict. Will you take side with your allies knowing that might be that one too many fight for the day? Will you sell one of your group members out if it meant saving some of your precious decaying health? Or Would you be the martyr up front, trusting, hoping that your team mates will have your back?
Making decisions. Is this fight really worth it? Can I afford it? I think it goes well with the global idea of competitive cooperation that they are trying to push forward. Might also make players think about why, sometimes, they'll want to take the darker approach. It is, after all, the Undead.
