Heat amplifiers and heat bonus's either are replaced with the concept of "Reduces the next level's cost by x%" or adds that to it.

That way if I get a 25% heat bonus, the next cost of upgrading is reduced by 25% (instead of needing 100 crystals for the 3rd selection, I'd need 75)

Since right now getting a "Bonus to heat" just means "I get to see it level faster, but for no reward seeing how I still need crystals."

I believe this would cause heat amplifiers, and the 25% heat bonus you get from forging sometimes, to be useful and not just "Oh goody, nothing changed."