Heat was once the system regulating the leveling of equipment, but has been essentially superseded by fire crystals.
So here is a new idea for them:

Factorizers
Factorizers are new type of equipment similar to accessories, with their own unique slots, that collect heat and create stuff from it.
'Stuff' can vary from hearts, powerups, to crowns or fire crystals and perhaps even more expensive items like low star weapons and armor.
Collected heat charges all factorizer you have equipped, and once fully charged they will drop their result on the ground.
However, the more expensive items require a very large heat investment, a heart factorizer needs a lot less heat than a crown or orb factorizer.

Factorizer keep their charge between levels, and even when you log off, though dying reduces their charge, making factorizers that require a lot of heat quite high risk/reward.

These would allow Knights to customize their loot drops, make their life easier with health factorizer, or bolster their crown drops.