I am aware that you can craft Fire Crystals at the Alchemy Machine, but this requires using higher-tier crystals to craft lower-tier ones (e.g., Shining → Glowing). What about adding a feature to the Forge that allows players to fuse lower-tier crystals into the next higher tier using the heat generated from previously played levels?

Example:

You get a new 5* weapon, but you're out of Rads and have tons of Shining Fire Crystals lying around. You go on a mission or play the Arcade, and after completing a level and progressing to the next depth, you max out the heat gauge on your new 5* weapon. The excess heat is then stored in the Forge Heat Storage.

The next time you can use your Forge, you could open it and fuse your Shining Fire Crystals into Radiant Fire Crystals using the stored heat.

Let's say the heat storage capacity is displayed as a percentage, ranging from 0% to 100%, and the conversion rate is 3 lower-tier crystals for 1 higher-tier crystal, with each fusion costing 10% of the stored heat.

The final equation for this example would be:

3× Shining Fire Crystals + 10% Stored Heat = 1× Radiant Fire Crystal

Of course, the conversion rates and mechanics of this system would need further discussion, but I think this would be a great way to make use of excess heat and lower-tier Fire Crystals, which I imagine many players have lying around.