Another day, another quick mechanics idea.
Hit Points (and all other injury models, to be fair) sometimes produce results that seem awkward or unconvincing. You get particular issues when it comes to description - either you describe all fights as a lengthy series of deflections and dodges until someone goes down, or everyone ends up taking gaping chest wounds that heal up in a few minutes. People are basically completely unhurt until they die. And it can be remarkably hard to actually kill someone who's largely defenceless, just because they have loads of HP and magically absorb the damage.
I can't remember what the exact chain of logic was that produced this idea, but I had an idea for a three-level injury system that would result in (let's be honest) a slightly different set of head-scratching. Partly it's because I do exercise myself, and I'm aware of the interplay between immediate energy level, ability to recharge, and actual exhaustion. Anyway, I had the idea, so I'm posting it.
The three levels here I'm going to call Stamina, Endurance and Reserves.
Stamina is what games typically call HP or something; it's your generic damage-soaking resource for games where regular fighting is expected, as opposed to those where if anything attacks you you'll just die. Here, it specifically represents your ability to deflect, avoid, discourage or (if appropriate) roll with attacks so as not to get injured at all, when you otherwise would be. Dodging, parrying, feinting, blocking, grinning and bearing it, all this is Stamina. However, Stamina can also be used as a more general stamina resource for things like long-distance running or carrying heavy stuff. It's expected that Stamina will be depleted easily in the short term, but that this doesn't cause any long-term problems. If your Stamina is depleted, you no longer have the energy to make those heroic efforts to defend yourself.
In D&D terms, successful attacks are those that force the target to burn Stamina as they avoid injury. A failed attack roll might still be parried or dodged, but the target is easily able to do so without strain. They could keep that up all day.
As well as attacks, Stamina might be depleted by rolling poorly on athletic feats, for example.
Endurance is essentially your general fitness. It represents your ability to recover your Stamina after bursts of activity. When you're taking a few minutes to recover, simply by not doing anything strenuous, you can spend Endurance to replenish your Stamina. When you've depleted your Endurance, you no longer have that ability to recharge, and when you exert yourself you just stay tired. Illness or lack of sleep reduce your Endurance; you might be capable of the same immediate effort, but take much longer to recover afterwards.
Finally, Reserves are your long-term fitness, related to your overall level of health. While Stamina can be regained in just a few minutes, Reserves allow you to recharge your Endurance over the course of hours. This is what you use when you're overcoming discomfort to just keep on marching, or withstanding the urge to stop and sleep. Unlike Endurance, you don't need an actual rest to expend Reserves, but you can only do so once per hour.
You could think of this as: Stamina lets you run a sprint, Endurance lets you run a race, and Reserves let you run a marathon. Or you could view Stamina as a single volley in the tennis court, Endurance as a game, and Reserves as a whole day's play. In gaming terms, Stamina would generally cover one encounter, Endurance several encounters (the classic 'short rest') and Reserves stretch to a whole day or even several days (the 'long rest').
Characters can also be differentiated by having different balances. A dashing, energetic character might have a lot of HP, but not much in the way of Endurance or Reserves. A slower, more resilient character might have fewer HP but enough Reserves to keep going steadily all day.
Actual injuries, where an attack connects solidly, would be covered by an injury chart of some kind. The use of Stamina for defence means that you rarely get injured (mostly that happens to one-off enemies), but when you do, it's significant. Injuries may reduce your Stamina, Endurance or Reserves - a painful or hampering wound might impair Stamina because it's hard to exert yourself, while sickness and internal injuries are more likely to impair Reserves. They can also have other effects, such as affecting ability to interact with objects, or perception.
In this model, sneak attacks are particularly dangerous not because they do loads of hit point damage - they don't - but because they ignore Stamina altogether. A successful sneak attack inflicts injury directly. As such, attacking a hampered or unsuspecting target is a very effective tactic, and being taken off-guard is dangerous.
One option is to treat all of these as pools of points.
Let's say arbitrarily that a character has, oh, 50 Stamina. They also have 5 points of Endurance, each of which can be spent during a minute or two of relative quiet to refresh 5 Stamina. And they have 2 points of Reserves, which can be spent once per hour to recharge 5 Endurance, but only one point per hour.
This means that if a character expects to take 20-30 damage in a fight, they can just about manage a couple of fights back-to-back, but really prefer not to. A few minutes' rest will leave them re-energised, but not completely fresh; in any given hour they can only handle up to three fights before they're worn out. If exertion is spread out over the day, they can survive up to five fights (or similar taxing efforts).
This is a little bit fiddly, of course. Some players would enjoy turning resources into other resources, others won't.
Another option is to use some stats to handle the way these work, which should be simpler. Let's use D&D as an example.
A character begins, again, with a nominal 50 Stamina (HP). If they spend a few minutes simply not doing anything strenuous (so exploration or talking is fine, just not fighting or running) after losing Stamina, they can regain Stamina equal to their Constitution score, which here functions as Endurance. Crucially, they can do this only a number of times equal to their Reserves, and can only do so once per hour.
Essentially this model cuts down on the points-converting aspect.
This interplays with other mechanisms in some ways I like. Constitution is already used to handle medium-term physical feats, so your ability to maintain intense activity determines your ability to maintain intense activity. Great. If you fail such a roll, you can take Stamina damage, which basically means that your medium-term endurance is fine, but you're a bit fatigued right now and need a bit of a break to recover. Great.
There are effects in the game that penalise or reduce your Constitution. As magic or monsters impair your overall fitness, your ability to maintain activity goes down because it's harder to recover Stamina, even though in the short term you can still defend yourself.
Let's take a couple of hypothetical characters.
A barbarian has a lot of HP/Stamina, due to a large hit dice, but they don't always take a high Constitution. You might build a barbarian with Con 10 who has 35 HP at 5th level. This character can defend herself effectively throughout a fight, but tends to tire out quite easily. She can only regain a quarter of her HP after a fight. You can view her as being an effective but inefficient fighter, perhaps, who exhausts herself with mighty leaps and ferocious parries.
On the other hand, let's build a dwarf wizard with Con 18. At 5th level, the dwarf has (in 5e) only 20 HP, about half what the elf does. However, the dwarf has enormous endurance, and not many HP to regain, so a few minutes after a fight he's as fresh as a daisy again. He's not that effective at defending himself, but he's fit enough that he can maintain that level fairly steadily.
I freely admit I don't know how you'd add a Reserves system to D&D 5e. You'd be basically scrapping the whole system and starting again here. I suppose my inclination would be to have a default level (say, 2 Reserves), and have a small number of additional Reserves available either as a racial feature, an optional class feature for appropriate classes, or a feat.
Sneak attacks here would probably come across a bit differently from the current model. Whereas sneak attacks traditionally just deal a lot of HP damage, you wouldn't necessarily cause a worse injury with a sneak attack than any other attack. Your backstab might still just cause a painful wound rather than puncturing a lung. The difference is that a sneak attack would have a reasonable shot at weakening or impairing a target early on, making the rest of the fight easier. I suspect in this model you'd want to reduce the circumstances where sneak attacks were available, otherwise rogues might be just too deadly - being able to ignore hit points entirely just by having an ally nearby is very powerful. I'd tend to exchange it for some lesser advantage, or just a version of the current HP-based sneak attack.
Anyway, I don't think is going anywhere in particular, so let's leave it at that.