Vatel will sell it, you can equip it in a trinket slot.
It gathers heat like equipment would do.
If it gets full it turns into a useable item.
You can use it whenever you want (i don't know if it should be un/bound or not, lemme see your opinions!) to heat up unheated stuff (maybe battle sprites, too???).
Obviously there have to be different sizes of heat flasks.
If the size is too big for low-level equip, it would be wasted heat (which is against the core-concept of this idea).
And if the size is too low it would get filled up too quickly (also wasted heat).
So multiple sizes please.

And there could be different uses, too.
If not used on equipment, it acts like a confetti?
A huge amount of the biggest heat flasks + other stuff + Drakon Pod = F2P Drakon Re-skin (please do it, please do it, please do it.).

Yeah, just an idea. I have several more. Don't wanna post all at once, though.

~ Peace