Most effects that copy spells let you change targets, and so people will expect that. This one doesn't, which I suspect will catch most players unawares. You can say "copy it" rather than "put a copy of it on the stack", so I suggest you use the saved words to say "You may choose new targets for the copies", matching the wording on Pyromatics.

This is scary, but I think you're doing a great job finding ways to incentivise the swarm approach. Nonetheless, keep a wary eye on this in testing, as I could easily see this being first-pickable at 4cc with a feint of . Common toughness values are pretty low at the moment, comparable to Scars block, where Arc Trail and Kuldotha Flamefiend are absolute blowouts. This has higher variance then either of those, but sits between them in overall power.

The thing I keep looking at here is that apart from what I’ve done to update (((Veteran Blastmage))), there’s no “Blaze” or “Fireball” analogue. Blitzburn can be huge and swingy, but it can also be a staggering dud. I’m honestly not sure how to cost it. More often than not, I suspect it’s going to be 3 or 4 points of splittable burn, which is a huge steal at either or , but in those cases where you draw two Blitzburn, four land, and one creature, the hand is pretty much a mulligan.

I am pretty happy though how the redefining of R/G is working out. Thanks for al the input.

Shouldn't it just say "copy it for each creature you control," rather than make that taboo reference to the stack?

Yes.