I'd like the ability to trade spare materials for heat. It would let higher end players trade out the several hundred beast scales and monster bones for something at least vaguely useful.

Perhaps something like the YPP system for trinket trade ins at black markets. Materials start with a base heat value, as people trade in materials like yellow shards, their heat value steadily drops. If a material isn't traded in, its value rises. This means a drake scale, which would almost never be turned in, would be a large amount of heat. Something that would almost constantly get traded in, such as yellow shards, would dwindle in heat value, leveling out somewhere very low.