Hi. I have made a system that strove for the same things you asked for. My goal was to have rules that captured the feel of wounds in cinema and fiction, and to use an understanding of how injury and death actually happens in modern medicine to explain why a PC can get 'defeated' without being killed.
Here's what I came up with.
Critical Hits and Wounds
Critical hits do not do extra damage. Instead, the attacker inflicts a light wound. If the target has 0 hit points after the attack's damage, that creature makes a Constitution saving throw (DC 15). If successful, the creature suffers a serious wound. If failed, the creature suffers a critical wound.
A light wound lasts until the creature heals any damage (such as by magical healing or spending hit dice during a rest), or is treated by a DC 10 Medicine check as an action, or is affected by the lesser restoration spell. (Example: dislocated shoulder, swollen black eye, twisted ankle, large laceration.)
A serious wound can be cured by a 3rd level or higher cure wounds spell. Alternately, whenever the creature takes a long rest it can make a Constitution saving throw (DC 15) to recover from the wound. (Example: fractured wrist, gashed eye, sprained ankle, arterial bleeding on a limb.)
A critical wound never fully heals naturally, and can be cured only by a 7th level or higher cure wounds spell, or the regeneration spell. (Example: severed hand, removed eye, arrow to the knee, deep abdominal bleeding.)
Wound Location and Effect
Regardless of the wound severity, the attacker rolls 1d6 to determine the effect:
1. Leg (or other mobility). The creature falls prone and cannot stand on its next turn. Until healed, its speed is reduced by half and it has disadvantage on any checks related to mobility.
2. Secondary Limb. The creature drops anything held in its off-hand or equivalent limb and cannot use that limb on its next turn. Until healed, it has disadvantage on any checks or attacks using that limb.
3. Primary Limb. As above, but for its primary limb.
4. Mouth. The creature cannot speak or bite until the end of its next turn. Until healed, it has disadvantage on any checks or attacks using its mouth, and in order to speak (such as to perform verbal components for spellcasting) it must spend its bonus action to be able to speak clearly.
5. Senses. The creature is blinded until the end of its next turn. Until healed, it has disadvantage on Perception checks and on ranged attacks.
6. Bleeding. The creature takes 5 damage now. It takes 5 damage at the end of any turn that it takes an action or bonus action (unless doing so ends its bleeding condition). If it has 0 hit points, it takes 1 damage at the end of each of its turns (which prompts a death saving throw).
As you can see, each wound has one immediate major negative consequence that lasts a round, and then a lingering effect until the wound is healed.
Helpless, Defeated, and Dying
When you are reduced to 0 hit points, you are helpless (you fall prone and are stunned, and if you have no hit dice left, you're unconscious; otherwise you remain conscious and can speak falteringly). This condition ends if you regain hit points.
When you take damage while at 0 hit points (such as if you have the bleeding condition), you make a death saving throw, which is a flat DC 10 check. (There is no special effect for rolling a natural 1 or natural 20.) If you succeed three death saving throws, you stabilize. If you regain hit points, reset the number of failed death saving throws to 0.
When you fail your third death saving throw, normally this means you are defeated. You remain alive, but have four levels of exhaustion. Thereafter, you cannot heal naturally, and after each week of rest you can make a Constitution saving throw (DC 15) to remove one level of exhaustion. When you have no exhaustion levels, your defeated condition ends. Most defeated characters retire.
When you fail your third death saving throw, if you have no hit dice remaining or if you are suffering from critical bleeding, you are dying. You remain alive for a period of time, usually no more than a few minutes, long enough to say some final words. You remain helpless, and cannot heal. Unless you receive the 5th level spell miraculous recovery (see below), you will die.
Raising the Dead
In place of existing spells that can restore the dead, use the following.
Miraculous Recovery - 5th level, casting time one minute. No component cost. One creature that's defeated or dying, or that died in the past hour, loses that condition and is restored to 1 hit point. If it was dying or dead, it has four levels of exhaustion. (After four long rests, it should be back to normal.)
Raise Dead - 7th level, casting time one hour. Costs 500 gp in components. As miraculous recovery, except it can affect creatures that died in the past 10 days. If cast on a creature that died in the past hour, the casting time is one minute, and the creature only has one level of exhaustion.
Resurrection - 9th level, casting time one hour. Costs 1000 gp in components. As raise dead, except it can affect creatures that died in the past 100 years. If cast on a creature that died in the past minute, the casting time is one action, and the creature returns to life with full hit points, all wounds removed, and no exhaustion.
Second Wind
If you are at 0 hit points but not defeated or dying, you can spend a hit die to regain 1 hit point at the end of your turn. This happens before damage from bleeding or other ongoing damage effects.
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Okay, I think that's pretty thorough. What do you think?
