(This is something of a Joesky Tax, since I haven’t posted any usable game stuff in a while.)
I have ranted before about “death spirals” in RPGs, where a character accrues penalties as they take damage. They tend to not be fun and are arguably less realistic too. (Although realism is the usual justification.)
In many movies and other fiction, you will observe the opposite. The hero becomes more effective as they take damage. In this spirit, I offer the cinematic anti-death spiral for D&D-style games. For every 5 points of damage a character takes, they gain a +1 to all rolls. And let’s give their opponents an equivalent penalty to their saving throws versus the characters spells and such.
This applies mainly to player characters, but the referee might also use it for “big name” NPCs.
(I should probably mention the “escalation die” that some game—13th Age?—uses, but I don’t really have anything to say about it.)