Fairly simple. If all of your equipment is at Max Level, any Heat you get should be converted into Crowns at the end of a level. The exact rate of conversion I'll leave to OOO since Heat is rather nebulous in its quantity, but it'd be nice to have it do something when you don't have anything to level up, and obviously nobody's going to go for a Heat -> Energy conversion.
I don't expect this to happen. That game already has issues with inflation resulting with there not being enough crown sinks in the game - I really can't see them adding something that would generate even more crowns...