There could be vendor where you can gather much like in gates, materials drops or what not, even excess heat to give boost to the gathering. And when certain amount of materials are invested in the vendor, you can purchase guild hall item or personal items. Example guild banner with guild logo and colors that you can carry or something.

Also Pets and *insert super graphics that make anyone jealous* items you can purchase with heat/material combo. By no means let these be purchased with energy/crowns to keep that market off the table.

Example I would love to get few chairs or something unique on guild hall. Also getting boss kill trophy in guild hall would be insanely cool. Example after 100 guild kills of Jelly King, we would get chance to buy purple crown in wall with our excess heat :)