Something I cooked up for deadlier combat spur of the moment:
Setting Rule: 5e Deadly Combat
- After total Hit Points are rolled at character creation the pool is split up into four Milestones equal to one quarter of the characters health. If the amount isn't divisible equally across the the four Milestones then any leftover HP is added to one of the Milestones, player's choice.
- Whenever a character takes damage that reaches a Milestone they take a level of Fatigue (See: Conditions PHB).
- Once a character is damaged past a Milestone they cannot be healed beyond that Milestone without a successful Medicine check (DC 15) before healing. Magical healing is the only exception to this rule and removes the effects of one level of Fatigue when healed beyond a Milestone.
- If a character takes enough damage to pass more than one Milestone they make a Death Save. If they fail they remove one Death Save Failure box completely. Characters gain one Death Save Failure box back after a long rest.
The idea is to have lasting and impactful results from extended combat and making players weigh the risks of getting into fights. Where HP in D&D is the abstract will to "keep on fighting" and not actual physical damage to one's person until they hit zero HP, each Milestone represents actually being hit, stabbed, hobbled, and bludgeoned enough times that your character isn't always in tip-top shape. This does make Level 1 characters more squishy, especially Wizards, but that's the point, innit?
Edit: This also seems to pass the Stork Stairs Test. You won't be killed but take enough damage from falling down sixty feet of stairs and you're likely taking two levels of fatigue (including the disadvantages that comes with them), down one Death Save box, and can't heal back up again without magical healing. Not dead, but you wish you were.