Game Mechanic: Negative Damage is Healing

I just had an interesting discussion with a bunch of 3rd party D&D writers. One wanted "heal" to be a keyword, and honestly, that's sensible. But I wanted to take a whack approach to this demand. This should work for any game that revolves around damage types, such as D&D, though it could also work with other systems that deal with numeric amounts of damage. To keep things simple, though, I'll use D&D 5e as an example system where this would apply.

Negative Damage

Damage can be reduced below 0, unless it is because of a negative ability score modifier added to the attack. When you take negative amount of damage, you regain that amount of hit points. Other modifications to the damage apply.

What does this achieve? You can flavor your healing spells in various ways, giving the imagery of how the healing works.

• The fire that cauterizes the wounds.
• Piercing damage applied straight to an acupuncture pressure point or something.
• The divine radiance that just heals because it's divine or something.

Not only that, but it suddenly matters also how you want to heal others. What if your group's warlock is resistant to fire damage? A spell that would deal -11 fire damage suddenly deals them only -6 damage (remember, it's rounded down, not just to zero!). Frankly, though, I assume healing spells would just let you choose what damage type the healing goes with.

Damage reduction would also be a lot better. A goblin hits you for a measly single point of bludgeoning damage, and your Heavy Armor Master feat reduces it to -2. Thus, you suddenly heal 2 hit points.

Of course, some adjustments would be necessary here, like solidifying the idea in players' minds that the hit points are not always just wounds. They can stand for stamina, luck, or some other things that PHB mentions.

Finally, absorption could become a proper game keyword. Immunity reduces damage to 0, vulnerability doubles it, resistance halves it, and absorption... negates it. Simple as that, AND it's already a thing in the game (see flesh golem).

Negative psychic damage is placebo, change my mind.
Xetorath Healer, by TSRodriguez (at least according to Pinterest, the original DeviantArt page for it seems to be down)

Anyway, time for me to get ready for my session, just wanted to write this up real quick before I forget the idea. Have a great day, everyone!