The prize wheel after clearing a stage has been around for ages, but outside of vending lootboxes, it hasn't really done anything for me. I vaguely remember getting a Mod Calibrator from it once, which was nice. It doesn't seem to be influenced by difficulty nor heat collected in a stage, just tier. Have you gotten anything cool from it?
I'm surprised there was a never a stage clear reward (prize wheel or otherwise) that gave firecrystals based on heat given how strictly tied rarities are to treasure boxes otherwise. Seems like it'd be an easy way to make them more consistent in the stages that could use it, like Danger Missions.
I think I've gotten a lockbox from the wheel...once? twice? Mostly it's just a bit of extra level-end fanfare. It can give you health if you're not full, which is nice, but it doesn't check if you still have your emergency revive first, so it can be kind of a waste (even if there's a good chance not much better would roll). I'm not even sure I've actually landed on the funny kleptolisk that presumably gives nothing; just watched it roll by.
Guaranteed crystals for heat collected at level clear would be neat, since it'd kind of compensate for heat not counting toward anything if your gear is all ready to be forged. I didn't play for too long before the forge update, but getting multiple levels on a weak item from a deeper clear or accumulating levels over a boss stratum was satisfying in a way forging doesn't allow. It wouldn't be the same, but at least the heat wouldn't feel pointless to gather.