One thing 5th edition has is a ranked Exhaustion system. A friend of mine kit-bashed it into a 'gritty wound' mechanic that actually makes PCs harder to kill, but makes injuries feel more visceral.
It would need to be a little different for PF, but here's how it basically works.
Exhaustion Levels
There are 6 levels of exhaustion.
- Disadvantage on ability checks (i.e., roll twice and take worse result, includes skill checks; in Pathfinder I'd probably make this just a flat -2).
- Speed halved.
- Disadvantage on attack rolls and saving throws.
- Hit points cannot heal above half your maximum.
- Speed reduced to 0.
- Death.
Wounds and Exhaustion
You don't track 'negative hit points.' The lowest your HP can go to is 0, and you remain conscious while at 0 hit points.
You do automatically die if you take damage from a single source equal to twice your maximum hit points, but normally you'll die over a few rounds, represented by accumulating exhaustion.
While you are at 0 HP you have two temporary levels of exhaustion (this replaces the 'staggered' and 'dying' conditions in PF). If your hit points rise above 0, this temporary exhaustion goes away.
Additionally, whenever you drop to 0 HP, or when you take damage while already at 0 HP, you gain one level of lasting exhaustion, which stacks with the temporary exhaustion. You also fall prone.
Whenever you take an action that exerts you while at 0 HP (basically any standard action), you must make a Fortitude save (DC 15) or else the pain causes you to fall unconscious and you don't complete your action. Either way, you suffer an extra level of lasting exhaustion after the action.
(Something like Diehard would let you avoid falling prone at 0 HP, remove the risk of falling unconscious, and instead have a successful Fort save let you act without taking additional exhaustion levels.)
So someone who goes to 0 HP the first time in a combat will end up with 3 levels of exhaustion. If they're healed, they'll still have 1 level. If they're knocked to 0 again they'll have 4 levels of exhaustion (2 real, 2 temporary). If they keep fighting despite the sorry state they're in, they'll get 5 levels of exhaustion.
Most people who are smart stay on the ground while at 0 HP and don't fight back.
You heal 1 exhaustion level with a night's sleep, or 2 with a full day's rest. Magical healing only removes exhaustion if you are already at full HP. Any magical healing used when you're already at full healing removes 1 exhaustion level for each die of healing (or for every 5 points of non-dice healing, like lay on hands).
It got a little more fiddly with mechanics for eventually bleeding to death if you were alone below 0 HP for a while (in a matter of days at 3 levels of exhaustion, hours at 4 levels, and minutes at 5 levels), but it never came up, since unless there was a TPK the party would stabilize you.
The tone was really set for how this mechanic would work when, in the first encounter, we were beset by zombies that grappled my character, tripped him, and began biting him and tearing at his flesh. I remained conscious and struggling even down to 4 levels of exhaustion before the rest of the party managed to destroy the undead. They'd already blown their healing trying to keep me alive, so I just had to stagger after the party, holding my throat closed with my hand. Fun times.
Now, for our game the GM paced encounters differently than normal D&D or Pathfinder, and it was more up to us whether to pick fights and how much trouble to get into. ZEITGEIST sometimes throws several encounters at the party in a row, so this system could force the PCs to fail just from exhaustion. But I thought it might interest you.
