So, amongst the posts about QTY of hit dice and taking injury out of the realm of attack roll chance, I thought I'd expound on my favorite hit points/wounds/injury system, which makes hit points actual meat points.
First - you have to concede that every creature is going to have hit points in a relatively narrow band. In 5e, a 1st level fighter has 1 HD and typically has between 10 and 14 hit points (let's assume STR 16 and CON 18). A 10th level fighter (let's assume STR 18 CON 18 and take the average of 6 hp per level) has 10 HD and 104 hit points. A commoner has 1 HD and 4 hit points. An orc has 2 HD and 15 hit points. An ancient red dragon has 28 HD and 546 hit points. A stone giant has 11 HD and 126 hit points. A black pudding has 10 HD and 85 hit points. A horse has 2 HD and 13 hit points and a lion has 4 HD and 26 hit points. While not an exhaustive look, what I'm trying to illustrate is that the range of HD tracks closely with hit points for humanoid-type creatures only, and we want to change that.
What if you were to base hit points solely on an algorithm associated only with your physical fitness (STR) and your relatively healthiness (CON)? Why should gaining "experience points" translate into being able to take more damage unless you've actually increased your fitness, stamina, and overall health?
My favorite system (converted to D&D terminology) would assign a "base" hit point number = (10 + STR + (2 x CON))/4.
1st level Fighter = 16 hit points.
10th level Fighter = 16 hit points.
Commoner (assume 10s) = 4 hit points.
Common Orc = 12 hit points.
Ancient Red Dragon = 25 hit points.
Stone Giant = 18 hit points.
Black Pudding = 15 hit points.
Horse = 13 hit points.
Lion = 13 hit points.
Alright - now we've constrained all creatures to essentially the same scale.
So - what if, instead of attacks result in subtracting hit points, you look at various multiples of the base hit points to determine injury results?
The multiples are:
Slight (1 hp)
Moderate (base hp)
Serious (1.5 x base hp)
Critical (2 x base hp)
So - the 1st level fighters wound thresholds are 1/16/24/32 and the orc's are 1/12/18/24.
Now - let's say the injury effects are as follows:
Slight: -1 on all checks (attack rolls, saves, etc.)
Moderate: Make a CON check or go into Shock. If you fail the check by 5 or more, become Unstable. -2 on all checks.
Serious: Make a CON check or go into Shock. If you fail the check by 5 or more, become Unstable. -3 on all checks.
Critical: Go into Shock. Make a CON check or become Unstable.
Shock: Lose your turn this round. For rest of the combat, any effect that would put you into shock makes you become unstable instead. At the beginning of your turn on each subsequent round, make a WIS check - failure results in being semiconscious and unable to take any actions beyond disengaging and moving at half-speed.
Unstable: At the end of each full round, your wound level increases by one until you go one level beyond Critical at which point you have bled out and die.
The beauty of this is it allows you to add hit location as an option, so you can change up the effects (such as making a STR check to avoid dropping your weapon if hit in the arm, a check to avoid falling prone if hit in the leg, reduction in movement speed if hit in the leg, or even a chance to go lights out with a critical wound level to the head.
You ask - isn't this going to be deadly, quickly? Not necessarily, because the way this would work is that taking a lesser wound than you already have will cause no additional effects. But, taking another wound of the same severity will increase your wound level by one level. What this does is make dragons as terrifying as they ought to be for PCs of all levels and "normal" monsters more of a challenge to less experienced PCs. It also makes PCs think before they decide to enter combat since it is easy to get into, hard to get out of unless you run away, and it makes trading blow for blow less desirable because even a lowly 1 HD NPC with a lucky enough hit can increase their chances of killing a PC. It also makes PCs rather reluctant to get into fights with those stronger than them to begin with.
Example using Orc vs. 1st level fighter:
Round 1:
Fighter wins initiative, attacks, and hits with longsword, doing 6 points of damage (I'm assuming STR 16 and average 4 damage on the roll). This is a slight wound to the Orc.
Orc attacks at -1, hits and does 9 points of damage. This is a slight wound to the fighter.
Round 2: Fighter attacks at -1 and does 6 points of damage. This is a 2nd slight wound to the Orc, making him now moderately wounded. He makes his CON check (at -2) so remains in the fight, but is -2 to attack.
Orc attacks (-2) and misses.
Round 3: Fighter attacks at -1 but rolls a natural 20. Lucky rolls of 6 and 8 on the dice, resulting in 16 points of damage. This is a moderate wound to the Orc. Since the Orc already has a moderate wound, he now has a serious wound.
The Orc makes a CON check at -3 and fails. He is now in shock and can take no action this round.
Round 4: Fighter attacks (-1) and misses (rolled a 2). The orc rolls his WIS check at -3 and spectacularly makes it, rolling a 20. He snarls, as he redoubles his efforts but misses on the attack roll.
Round 5: Fighter attacks (-1) and hits, for 6 points of damage. The orc doesn't really feel this as more than a flesh wound since he is already moderately wounded and tries to make his WIS check and fails. He is now light-headed and woozy due to some blood loss (and the sting of actually having been cut multiple times by the hated human's sword) and attempts to stagger away and get out of the fight.
Round 6: Fighter goes after and attacks the helpless orc, slicing open his abdomen (critical) and hits for 18 points of damage (double 8s!!) The Orc has sustained another serious wound. Since he already has a serious wound the second one makes his injuries critical. He goes into shock. He tries to make his CON check (at -4) but fails and he becomes unstable as his intestines spill out onto the ground and pinhole leaks of blood start pouring out of his abdomen. The orc falls to his knees, trying to put his guts back in.
Round 7: The fighter puts his boot into the orc's chest and kicks him to the ground. The orc can only lay there and pray to Gruumsh. At the end of the round, his wound level increases one level and he expires in a widening pool of blood.