There have been repeated ideas in Light or Darkness to have this effect. However this is sufficiently different.
When would you use this?
Other than
Darkness or Death mark to use nightfall
Darkness or Death mark to use Holy Light
Water to use Flooding
u forgot air for sky blitz, and another note which i just thought of, this card would most probably go really well with fractal since fractal consists of copying the exact card onto your hand (besides mutated creatures) maybe as suggestion, the element of the card would probably remain the same in your hand if your fractal it?
but i see your point, my "counter argument" would be "Shard of Readiness", since about 60% or less of the creatures in the game have abilities to reduce their costs to zero, but nevertheless out of those many abilities, there would only be a select few on which people would generally use SoR for, such as druid´s "mutation|improve" ability, so a card only useful for some creatures, which makes me think that this card would have a similar "total usage" as Sor at least until card pool becomes bigger.
What exactly changes? Just the element of the card, or anything more than that?
If it changed something like the cost, then that would be something to look into; what about if it was Reverse Timed? Would it return to its initial element, or would it keep its newer element? if it was changed into an Air card, does it get 'airborne' passive? etc. etc.
It's missing a few of the finer details IMO, but otherwise it seems like it could be an interesting card.
i slightly agree with you on that, although to answer your question, only the element of the card changes and it stays that way, and the only useful thing that changing the cost of the creature does would have to be linked with "fractal" in the case of changing both its cost and element as you suggested. airborne passive wasn´t considered but perhaps by changing into air such an effect may occur.