I got switched to driving truck overnights, so no time to play but lots of time to think. It's still piecemeal since I can't write stuff down while behind the wheel and I'm not sure if it's been done somewhere else before. Here it is anyway:
Max HD = starting HP, Con+mod = Stamina. Seeing as AC is supposed to be an abstract idea representing not just armor but the ability to dodge, block, absorb and HP is an abstract representing the ability to fight, I feel that even a missed attack would be doing something to a character regardless of the roll. Exception, Critical Miss is a true flub and no impact on the defender. PC characters cannot (edit
fall below 1 HP unless Stamina has been depleted. NPC do not gain this trait unless they have 2 or more HD. If an attack misses do half damage to Stamina only, on a hit can divide full damage at will between HP and Stamina. On a Crit, full damage to both. At level up add min HD to HP total and max HD to Stamina. If Stamina is at 0 but at least 1 HP remains, either/and/or disadvantage on attacks, grant advantage during defense. Can use your action to gain Second Wind once per encounter, restoring half max Stamina. ooo Just thought, on a Crit Miss do half damage to your own Stamina...
Haven't crunched anything and can't playtest since I'm on the road when everyone else is having fun, but please feel free to punch holes or add/subract ideas for this since I'd like to make it workable. Also, please fell free to use it if it interests you.
EDITED for clarity