I'm guessing you need a lot of instants and so you offer to have them take creature slots in your Limited deck? Because a creature with a built-in spell would be a lot easier to understand, write and represent.

Mmm, this does want to be Jund Sojourners or Gempalm Incinerator really, doesn't it?

But a "creature with a built-in spell" (whatever that's supposed to mean amongst all activated/triggered/ETB abilities) is at best commonplace and not exactly the first thing that pops into your head when you think Izzet, is it?

Having creature type Weird would help with the Izzet associations. I assume it would be a discard ability like Ghost-Lit Raider or Jiwari, the Earth Aflame. Those would be nombos with Izzet in general because of how much Izzet love instants and sorceries, but the Suvnica project is a counterfactual replacement for the Ravnica guilds in a world where none of the Ravnica guilds existed, so it wouldn't feel as bad.

How about a kicker that leaves a Weird token if you paid it? That way the spell does work as a spell (and triggers relevant Young Pyromancers) and fits the creature need instead of being emergency Gray Ogres.

You'd probably want them all to be the same size, though. But you do save having tokens with counters in commons.

You're probably that this would be a better way to go about it, if only because the connection between the token stats and CMC is likely too subtle.

Soo, I've decided Incarnate and Summon (Shiny Umbreon's proposed versions) are too different that the new versions needs to be a separate mechanic. I also came up with a wording that replaces the spell effect instead of being an activated ability.

I still think it's weird, because it does feel like an activated ability, just like morph. But it's better that it at least works with relevant things.