I have a suggestion which would make sense in Spiral Knights.
When heating up a weapon, if you don't have enough of required fire crystals, just use a different type of crystal instead. No additional payment required. But exchange rate is not good. Example:
When upgrading a 5* weapon with a 4* crystal (Radiant replaced with Shining), take Shining fire crystals at 2x or even 3x rate (up to 500 crystals for one level!), but at no additional cost. When using a higher-level crystal for lower-level item, replace them at 1:1 ratio, use same amount of Radiant fire crystals that you'd use Shining ones.
This way players are recommended to use proper items, but are not so severly punished for not having the correct item.
Alternative suggestion: combine all fire crystals into same kind and count higher-level crystals as simply more crystals. So if anyone has 10 radiant, 10 shining and 30 glowing, and they're converted into "heat crystals" at 1 item per star level, it'd make 50+40+90 crystals for a total of 180 crystals, with higher-level 5-star item requiring 750 crystals or so for "100% success" upgrade. Simple math.
This is absolutely required since no matter how you change logics behind crystal drops, someone will be dissatisfied.
I remember in the past tons of radiant crystals fell in FSC, while no Shining ones dropped in same zones. And people with 4* gear were unhappy (why? they should've farmed tier 3, stratum 5 instead!). And Shining crystals replaced all but few Radiant crystals. And now I can't upgrade 5* gear simply because Radiant crystals don't seem to drop at all (they do drop but I never have enough for even 1 item, they're just too rare).
On a side note, I have over 1000 shining fire crystals. I had zero a week ago... and I can't upgrade my weapons because I can't combine 4* heat crystals into 5* heat crystals, only break'em down. Which is a bad game design, IMO. Sadly, I'm not a game designer yet, I'm programming in a different sphere (crawling toward making games, slowly).