So, if I recall correctly, in the original 1st edition D&D, Gygax had written that a round of combat between two combatants was actually an exchange of blows and the loss of hitpoints wasn't any one specific hit but the overall outcome of a series of attacks on the targeted combatant.

At the same time, today, we see a lot of game systems featuring regenerating hitpoints to save time, and part of this is also no doubt to get around anomolies like the idea that the first level fighter might get a day of bed rest and recover 3 HP, representing a full recovery, but the legendary 20th level fighter will show almost no noticeable improvement in health from the same day of bed rest and 3 HP recovery because his overall "gas tank" of hitpoints is many times the size of the first level guy. Hitpoints as some kind of gas tank of trauma has some inherent logical problems and in some ways does pointlessly slow down game progression.

This is also not to mention comical, narrative-breaking things like high level characters being able to absorb ballista shots to the chest, swim through lava, or survive mountaintop falls due to large amounts of hitpoints.

So, how about handling it this way:

1.) Hitpoints represent fatigue, psychological resources (mental toughness and combative attitude), and pain tolerance for minor injuries like contusions and non-dangerous flesh wounds.

2.) Constitution points, rather than hitpoints, are used to track physical trauma. So if you unwittingly step on punji spikes, your hitpoints don't come into play because you're not sparring with the punji spikes, but rather the points of damage are applied against your Constitution score, which is temporarily reduced by the amount of damage done. (I know I'm not the first person to have come up with this...I've read house rules like this before.)

3.) If your Constitution hits zero it means you've taken lethal trauma, so either the character dies, or if the gamers want to have some kind of bleed out and resuscitation system such that the character has simply gone unstable, obviously they can do that.

4.) When someone has a reduced constitution score, they get proportional penalties to all actions across the board, since it's hard to do things when you have physical trauma.

5.) As long as a character is not exerting himself, hitpoints come back by a certain % of the full amount every 5 minutes of game time. This % is affected and reduced if Constitution is down due to trauma, by psychological state/morale, by nutritional status, etc.

6.) Successful backstabs or undetected sniper shots also deal Constitution damage and ignore hitpoints. Anything where the victim is totally unaware and effectively gets sucker punched goes straight to Constitution damage.

I think anyone who has done combative sports or contact team sports can empathize with engaging with someone when you're fresh and have an aggressive mindset, versus if you are in a negative mindset, you are exhausted, and your limbs are going stiff due to deep bruises.

Going low on hitpoints represents becoming exhausted. Hitting zero hitpoints or below is like when an exhausted boxer takes the punch to the head or liver taht drops him to the canvas; the character goes down because he/she was too exhausted to be able to defend properly. Any melee attacks directed at a character with 0 HP or less automatically hit. Any overflow past 0 HP is deducted from Constitution. The underlying premise is that any serious attack with something like a sword or warhammer can take down even a linebacker with a good direct hit.

To minimize complication and maximize combatibility with existing D20 systems, armor would still work the way it traditionally does (it makes it less likely you will lose hitpoints, which isn't a discordant result as it would cut down on contusions and minor cuts) but it would also absorb Constitution damage due to trauma. Maybe to keep things simple all armor can absorb one point of constitution damage for each armor point it grants. However the armor gets damaged when it does this so each point of constitution damage it absorbs also reduces the protection it grants by that much, like if someone stood over your prone helpless form and smashed down on your chest with a maul, breaking your breastplate. Thus armor will need to be repaired if the user gets to zero hit points and then absorbs a good hit before it provides protection again.

Thoughts?