In D&D 3.5e, we had Ability Damage. Things (especially poison, but also other stuff, especially various creatures) could deal damage directly to an ability score -- if you take 1d6 Strength Damage, your Strength score goes down by 1d6 (so your Strength modifier correspondingly goes down by about half of 1d6).
If any of your Abilities reach 0 from damage, you are, depending on the Ability, paralyzed (a physical score), unconscious (a mental score), or dead (Constitution). If you take an 8-hour sleep, you heal damage in each damaged Ability by 1; if you take a 24-hour sleep, you heal each by 2. You can also take Ability Drain, which is basically the same but it doesn't heal without magic.
The designers of 5th Edition, of course, judged this all absurdly fiddly and mathy -- not only do you have to track each of your current Ability scores separate from your actual Ability scores (plus keep track of how much is Damage and how much is Drain), you also have to do the (x-10)/2 (rounded down) thing for each one to calculate your current and actual Ability modifiers -- and, probably rightly and correctly, dispensed with the whole thing.
I do find its loss very mildly unfortunate. It did allow a fair bit of differentiation between monsters. This guy just paralyzes you, that guy gradually makes you more sluggish until you're paralyzed, the other guy makes you dumber until you're paralyzed, and so on.
To the rescue: the OSR stylings of Swedish TTRPG Dragonbane!
In Dragonbane, each Ability score can have or not have a Condition. If you have a Condition in an Ability score, every time you roll that Ability for anything, you simply roll with what in 5e would be called Disadvantage (in Dragonbane, it's called a "Bane").
Porting that back to 5e, we could, while maintaining a high level of simplicity, have 3 (or even 4) levels of "ability score damage":
- Regular, undamaged (roll normally)
If you then fail a save vs Ability Damage: - Damaged (all rolls with that Ability have disadvantage)
If you then fail another save vs Ability Damage in an already-damaged Ability: - Paralyzed/Unconscious (or, if Constitution, unconscious and making Death saves)
We could also introduce some sort of blessing/boon/etc that goes the other direction: - Blessed or something (all rolls with that Ability have advantage, with the side benefit that you have an extra level of buffer against Ability Damage to that Ability -- failing a save vs Ability Damage when you're blessed in that Ability just brings you down to normal)
