I'm sensing more than a little exasperation from certain quarters regarding my plea for fatigue to be a recognized component of DnD combat.
For those "in the know", it will be plain that I'm no game designer. I don't have the critical mind for it. Nor the patience for endless stress-testing, to ensure a game component or system is sound.
I leave it the rest of you in the OSR community (the ones who possess real creativity and intelligence) to come up with novel solutions.
As mentioned earlier, hit points were re-purposed by Gygax to encapsulate many disparate elements of combat effectiveness, not to simply represent the capacity to absorb wounds. Luck, fate, skill, endurance, willpower, concentration, and stamina are just a few elements encapsulated by the hit point pool, in addition to physical damage. Despite that, very few, if any DnD, players utilize the hit point pool to its full potential.
In traditional DnD combat, the only way that your hit point pool can be diminished is if you absorb an attack from an opponent. But it is acknowledged that this pool represents skill, endurance, concentration and strength of will, in addition to fate, luck (the avoidance of otherwise damaging blows) and capacity to absorb damage.
If the hit point point pool represents skill and endurance, why can a player not exhaust some of that hit point pool to inflict additional damage on an opponent? Effectively, the player would be expending some of their skill, and/or accepting some combat fatigue, in exchange for inflicting additional damage.
This sort of approach would not require any additional book-keeping, other than the book-keeping that every player already accepts, ie. tracking their remaining hit points. It would also not contribute to that nay-sayer phantom, called "the death spiral," since the expenditure of the hit points would not affect the character's combat effectiveness, merely reduce her endurance (hit point) pool.
I leave it to brighter minds than mine to come up with a way to implement this, although off the top of my head, here is one approach.
During every round of combat, each player can declare, after a successful roll to hit, whether they wish to expend points from their hit point pool (become fatigued) in order to inflict additional damage. The amount the player can expend from the character's hit point pool, in any round, is tied to the character's level and class.
For example, Magic-users can expend a maximum of 1d4 hit points, per level, from their hit point pool. So a 5th level Magic-user could expend a maximum of 5d4 hit points from her hit point pool. In exchange for the assumption of that fatigue, she can inflict double the number of dice of extra damage.
So if a 5th level Magic-User accepted 5d4 of hit points of fatigue from her hit point pool, she would inflict 10d4 of additional damage on her opponent.
Similarly, Thieves would use a d6, Clerics, a d8, and Fighters a d10. In every instance, whatever number of dice was chosen (to a maximum of that character's level) for the assumption of fatigue to their hit point pool, twice that could be inflicted on one's opponent.
It should be obvious the potential effects of such a rule. First, character heroics: that a character can "go down fighting" by selecting an assumption of fatigue damage that would take them to 0 hit points or below. For example, a 5th level fighter, with only 8 hit points remaining, might select 5d10 of additional fatigue, in order to do 10d10 damage to her opponent, knowing that, while she might take out the monster, she will likely die in the attempt.
Second, there is a possibility that the number of hit points lost to fatigue will be greater than the damage inflicted. That's the risk the player takes, in choosing to make this fatiguing attack.
Third, in order to make this sort of rule palatable to the players, the recovery of hit points needs to be increased. I like the idea mentioned elsewhere of 1 hit point per round or per hour, rather than day, with, say higher recovery rates for sleeping, eating and drinking strong drinks.
That is a amateur's stab at the idea of combat fatigue. I'm sure others have better approaches to this problem. The point is, there is a way to recognize and account for combat fatigue, without adding significant complexity to DnD battles.