My HP Philosophy
In 5e, if you have any hit points left, you're still basically uninjured, just a little sore, bruised, and scraped. Heck, even when you're at 0 HP, you might still just be a little jostled, like a boxer who gets knocked out and takes a few seconds to struggle back to his feet.
My Conundrum
A PC in my game got scared by a ghost, and aged 30 years. The player threw a bit of a fit because, in this game you can be buried in lava and climb out, then be fine 6 hours later. You can never sprain an ankle, break a bone, or get a concussion (things which you can actually heal in reality), but you can be magically aged three decades, a condition which can only be reversed if you cast a 7th level spell within 24 hours. Where's the middle ground?
Should there be a middle ground? I think yes, the game should have some element of enduring debility, but it should capture the feel of high fantasy adventure.
My Observation
In heroic stories, characters are either too injured to fight (in Diehard, right after he gets glass in his feet), or they're basically unharmed (every scene afterward, those horrible wounds in his feet never seem to have an effect).
Only occasionally will a character have a Narratively-Significant Injury (Inigo Montoya with a dagger in his gut, Boromir full of arrows, etc.). If you have a NSI, you're probably slowed down a bit, and your opponent might exploit it by punching you in the wounded spot. Otherwise you grit your way through the pain and can save the day just fine despite your suppurating gut wound.
Since these injuries are ultimately narrative, I'm tempted to resolve them with narrative mechanics. In particular, taking the Aspects mechanic from FATE. 5e doesn't have fate points, but we'll make do.
My Proposed Mechanic
Set-Up. You have the usual hit dice and HP, and you regain full HP and half your hit dice with a long rest. This is normal. Remember, HP are just verve and grit, and you recover that pretty fast.
Step One, Knockout. (This replaces normal death saves.) Whenever you drop to 0 HP, immediately make three d20 rolls (unmodified, DC 10) to see the consequence of the hit that dropped you. If you succeed all the saves, there's no lasting consequence. If you fail 1 check, you suffer a minor debility. If you fail 2 checks, you suffer a major debility. If you fail 3 checks, you're dying.
Next, roll a Con save (DC 10). If you succeed, you're semi-conscious, are aware of your surroundings, and can communicate, but are helpless until you receive healing. If you fail, you're unconscious until you receive healing. If you beat DC 20, you're able to keep fighting. You fall prone, but you stay at 1 HP. (This is known as the Boromir Rule.)
Step Two, Consequences. If you're dying, you lose one hit die each round. When you run out, you die. (Note, this can happen even if you're still up and fighting due to beating DC 20 on your Con save.) However, if you receive any healing before you run out of hit dice, you are no longer dying, and even if you actually die, if you receive any magical healing within 5 minutes, you are restored to life, albeit with a major debility.
If you have a minor debility, the GM chooses either arms, torso, or legs. If arms, once per scene the GM can force you to reroll an attack roll, save, or check that involves your arms. If legs, once per scene the GM can grant an opponent advantage to attack you or make you reroll some sort of agility-based check. If torso, once per scene the GM can have you take double damage when an effect damages you.
If you have a major debility, in addition to the effects of a minor debility you move at half speed, and if you take any reactions you cannot move at all on your next turn.
Step Three, Recuperation. Major debilities are reduced to minor after five extended rests, and minor ones are fully healed after a further five extended rests. When you take an extended rest, if you benefited from any cure wounds spells in the past day, add the level of the highest-level such spell you received to how many days the rest counts as. So if you have a major debility, and receive a 7th level cure wounds, when you rest it counts as 8 days worth of extended rests for the purposes of recovery (reducing the condition to a minor debility, but still needing 2 more days of rest to fully heal). Multiple healing spells don't stack in a given extended rest.
The idea is that if you get a debility, it will stick around for at least a day, and magical healing just keeps it from lingering long-term.
What do you think?