So in addressing
@Mort's comment above does anyone have any ideas around making death spirals more interesting, perhaps porting something over from a different game. In 4e some creatures received powers when in the
bloodied state - I was thinking the same could occur when one entered the death spiral.
The obvious go-to idea is a refresh of the Inspiration Point (Desperation Point).
Death
spirals, definitionally, can't be made into a good thing. I know you understand the concept, but just to lay out my reasoning. The very concept of a death spiral is that as a bad thing happens, it becomes harder for good things to happen and easier for worse things to happen.
To actually clumsily port this over to an hp example, it would be as if you took more damage the lower your hp is. For example, if we do intervals of 10, and a player has 110 hp, you would expect you can take 11 hits. But, if they take +3 damage every time they cross a threshold (representing the harder time they are having dealing with blows and the increasing damage to the body) then you would get
110 -10 = 100
100 - 13 = 87
87 - 19 = 68
68 - 22 = 46
46 - 31 = 15
15 - 37 = DEAD
Taking 10 damage each time results in death within 6 blows, instead.
So, Death Spirals can't be made interesting, but perhaps they could be mitigated and the effects made more interesting. Something like a boon or boost for hitting 50% or 25% health does something like this. But, I think debilitating injuries like "broken leg, half speed for the next six months with magical healing" simply don't support any sort of counter-balance. The best I could think of would be to create bonuses for being super healthy and in shape, but in practice that just acts as a boost and makes players even more likely to immediately over-treat any injury.
Maybe though, some sort of meta-currency. This goes way beyond DnD's normal rules, and I know some people despise meta-currency, but I could see something like this working. I'm going to call them Action Points, because that is a thing in DnD already to a degree.
DM: The Ogre cries with their club, as you raise your shield to defend. Jim, you have a choice. If you accept a "Broken Arm" from the critical it will prevent you from using that arm unless you take disadvantage on everything from the pain. That will last for a week. But, you'll get three Action points. If it is just sprained, the penalty will last for only a day, but you'll get only one Action Point. Or, no penalty, no points."
I know this sort of interaction would slow the game down, and it could be weird for some people, but it does two important things. 1) It makes the injury the players choice. This is HUGE. If you force these sort of decisions on players, they will resent it, but if you make it a choice, a gamble between taking a penalty and hoping for a reward, then they can get invested in it. 2) It balances the penalty by giving them something of value. I'm thinking of Action Points because I'm thinking of cinematic and heroic actions. But it could also be a system like Piety. The idea is, it is more heroic and awesome to be severely injured in the course of combat than it is to be minority hurt in combat. Batman getting stabbed isn't super exciting. Batman getting stabbed... then spending the rest of the issue still fighting despite him bleeding out is a lot more notable.
But, this is just a 30-second brainstorm.