I like thinking about alternative ability scores for D&D-like systems. Taking a bit of inspiration from Call of Cthulhu's Sanity score, here's an alternative take on character survivability.
Characters have Hit Points. For the sakes of a D&D-like system, this can range from 1 to 20, but is initially determined by rolling 3d6 modified by Constitution. A character with 21 Hit Points (theoretically possible only by rolling 18 HP and 18 Con) is a Conan-type superhero, as close to invincible as it comes. A character with 1 Hit Point is as close to death as can be imagined. A character with 0 Hit Points is dead.
There is no such thing as a Hit Die. Maybe Fighter-types roll highest 3 of 4d6, and Magician-types roll the lowest 3 of 4d6.
When a character takes damage, they must roll a Hit Point check, 1d20 under their current Hit Points. On a failure, they take a high amount of damage. On a success, they take a low amount or none. This may be expressed as '0/1' - for some minor nuisance - or something like '1d6/3d6' for a deadly threat.
If a character takes more than one-third1 of their current Hit Points in damage, they must make a Constitution check or suffer a lingering wound.
If a character gets a full day's rest, and does not have any lingering wounds, they may make a Hit Point check. On a failure, they gain one Hit Point. They may not do this, and make a Constitution check to recover from disease, on the same day, meaning that disease prolongs healing, and wounds prolong illness. Or maybe Hit Points are entirely non-renewable, in which case characters will not live long...
Hit Points do not scale with level in this system. I'd suggest using this alongside an armour system where better armour reduces the damage taken, rather than one which reduces the rate of hits. This allows for diegetic advancement.
This isn't entirely compatible with my earlier thoughts on wounds. It's not fully thought through either. But there's probably some merit in it somewhere.
