I've been thinking about a similar mechanic. Let's call it Drain. It has levels like Exhaustion, and stacks with 2024 Exhaustion levels. However, it has its own more complicated little subsystem, which I developed to achieve the following: (a) I don't want Drain levels to accumulate too quickly, or an encounter with multiple monsters that inflict Drain and some bad rolls could kill a PC; and (b) I want to discourage the tactic of waiting for a PC to reach 0 hp before healing them. Drain makes it more expensive to do so.
Bloodied is already defined in 2024, but I want two more definitions of hp levels. Let's say a PC who has 75% of their hp or fewer remaining is Scratched and a PC who has 25% of their hp or fewer remaining is Battered.
A PC who is not Scratched can have at most 1 level of Drain.
A PC who is not Bloodied can have at most 2 levels of Drain.
A PC who is not Battered can have at most 3 levels of Drain.
A PC with any levels of Drain has Healing Reduction equal to 5 times the level of Drain. Healing Reduction is deducted from any healing the PC receives. Hence, if a PC has 1 level of Drain and receives 10 hp of healing, the PC only regains 5 hp.
In addition, a PC with 3 or more levels of Drain has Healing Resistance, which halves the amount of healing received by the PC after Healing Reduction is applied. Hence, if a PC has 3 levels of Drain and receives 25 hp of healing, Healing Reduction cuts it to 10 hp and Healing Resistance further cuts it to 5 hp.
More than 1 level of Drain also affects your hp maximum.
A PC who has 2 levels of Drain has their hp maximum reduced by 25%.
A PC who has 3 levels of Drain has their hp maximum reduced by 50%.
A PC who has 4 levels of Drain has their hp maximum reduced by 75%.
One level of Drain can be removed with 5 hp of healing. Healing Resistance applies, so 30 hp of healing is required to reduce 4 levels of Drain to 3 levels of Drain.
However, the PC's hp needs to be at a minimum level to further reduce Drain. It is the reverse of the PC not gaining more levels of Drain until their hp are reduced.
To reduce 3 levels of Drain to 2 levels of Drain, the PC must not be Battered.
To reduce 2 levels of Drain to 1 level of Drain, the PC must not be Bloodied.
To completely remove Drain, the PC must not be Scratched.
While a PC has any levels of Drain, they do not recover any hp from a Long Rest. They do regain Hit Dice, which they can spend to recover hp and reduce Drain.