I recently introduce Evil Lincoln's Strain/Injury variant for HP in my games (if you aren't familiar with it, it may be a good idea to search on the forum), and my players have taken to it. They don't mind the extra complexity because A) It also gives them the mercy of quickly regaining at least some of their HP back and B) It makes combat a bit more flavorful.
I'm interesting it taking this idea a little bit further with my group. I have suspicions sometimes that my players (all roleplayers first, power gamers second) get tired of combats that become a repeated strain of miss/hit/damage. Most of the time I'll have some sort of quirk going on that makes the battle more interesting (ex:attacked by dire wolves in a misty forest, smart players climb the trees and use their ranged weapons while the melees do their thing) but it doesnt always work out that way. In one of my first adventures as GM, one of my friends who was new to the game kept trying to make called shots to mix things up but was frustrated at how hard they were to do.
Another concern I have is I want players to be less at the risk of dying, but more at the risk of permanent physical consequences (in my games peoples almsot never can be ressurected, so things can get super risky).
So here are two ideas of mine:
1. Instead of increasing the damage on critical hits, which is what most often causes player death, I would rather have more specific injuries based on the type of damage that hinder the player in a unique way in combat. A given damage type may have more than one possible injury type, which could be rolled for. Weapons that have a x2 crit multiplier would deal normal damage and have 1 tangible injury, x3 weapons would do normal damage and cause two injuries, x4 would deal normal damage and cause 3 injuries. Some injuries can stack. All of these are removed if the damage that causes the injury is healed either magically or mundanely. Examples:
-Disabled Limb: Player cannot use this limb for strenuous activity (cannot wield weapon if arm, movement speed reduced if leg). Slashing, Piercing, and Bludgeoning weapons could cause this injury. Roll for the limb. If the same limb recieves the injury twice, it is completely broken until healed. If 3 times, the limb becomes severed/detached.
-Deafened: Penalty on all perception checks that rely on sound, penaly to AC if flanked, spells with verbal components have a chance to fail. Sonic, Force, and bludgeoning damage can cause this injury. This injury stacks intensity.
-Sight: Penalty to perception that relies on sight, All targets have 20% concealment, spells with somatic components have 20% chance to fail. This injury stacks intensity. If it occurs 3 times, the player is permanently blind. Slashing, Piercing, Fire, and Electric damage can cause this injury (for the first two, it's just getting blood in the eyes. For the others, it the intense light/heat)
-Bleed: The character takes bleed damage (1 per round, equal to damage dealt). This injury stacks intensity. Slashing and Piercing weapons can cause this injury.
That's the basic idea. Frostbite for cold and degree burns for Fire are sorts of things I'd make as well.
2. I would also like to change how called shots work with this adjustment. Since HP does not represent physical harm exclusively (using Strain/Injury), Called Shots could deal no HP damage and instead cause just the associated effect. This would make them less powerful but easier to perform, giving players more options. Pretty much no one wants to make called shots with penalties that high.
These are my ideas. I could use any critiques or suggestions, I'm not a person that will just implement something willy nilly.