See Dimir Confidant.

This is probably unprintable too.

Izzet sooooo wants to be Johnny but designing Johnny mechanics is hard. You want cards that make the player feel clever. Both replicate and overload have the problem that there's not actually a whole lot clever to playing them, once you've got over the ingenious of overload's rewrite-text thing.

But it's hard to make ANY mechanic like that, when the Johnniest thing is finding weird quirky stuff that's not specifically designed for Johnny.

Kaladesh was successfully very Johnny, at the cost of being a bit broken in places :) Contraptions were moderately Johnny :) But most of that was having many individual johnny cards.

Imprint is SUPER Johnny, but I don't think it's a whole guild's mechanic.

This at least makes you feel lucky, which is Izzet even if not especially Johnny.

Part of the trouble is, Johnny tends to love finding decks, but weird decks too often turn into "a bunch of scry and a weird combo" which isn't especially Johnny to play. The best decks are something like "a bunch of enablers that let you hold the fort and something that works surprisingly well with them." If you don't get lucky enough to find a whole untapped theme to build your deck around.

What else could I look into? Is there any text-changing mechanic that could exist in higher numbers? Or something like imprint, is there a way of encouraging variety of izzet decks? Something like, creatures which let you search out one particular instant/sorcery from your deck which you've built the rest of your deck to work with. But not just "one that's broken in multiples"?

Have you considered different variants on uncertain outcomes? More clash than coin flip?

Good question, but it feels like same effect again is most likely to not feel bad when you don't get it.

I agree it's very hard to design a Johnny mechanic. Something open-ended like Imprint is ideal. Something like text-rewriting or copying feel quite Johnny except I rather suspect there's no sensible way to do those at common.

What does that leave? I wonder if it's possible to have an Izzet mechanic that can sit on common creatures yet somehow lends itself to open-ended combos. I'm thinking something like Splice (but ironically overload rather interferes that future possibility), or a way to get back instants and sorceries (but without leading to too much repetitive gameplay), or a way to make artifacts Johnnier like Quicksmith Rebel and Quicksmith Spy (again, not really common-suitable)...

Maybe a variation on Conspire (Burn Trail, Ghastly Discovery) where it sits on the creatures rather than the spells? Not as OTT as Wort, the Raidmother (much though I love her) because it'd be written on the individual creatures themselves. Like Ondu War Cleric or Zada's Commando but with the effect always being ", Tap another untapped creature you control: Copy target instant or sorcery spell you control." (Optionally adding "that shares a colour with this creature", but that makes it less Johnny so I wouldn't bother.)

That has the drawback that it doesn't matter much which creatures you put it on. Except that they lose the ability to do it when they attack. Hmm.

Flip a coin.
Storm.
A marriage made in madness. Wait; why isn't this red?

Because even if it flips coins, red doesn't get to do things outside its colour pie like drawing cards without discarding first.

Ah, yes, "copy any instant or sorcery" is a very promising Izzet approach. Basically rewarding you for cramming your deck with instants and sorceries, but also finding the most appropriately synergistic ones. Or having one or two key ones you want to find.

You're right that it's odd it doesn't really affect the creature at all, but that could work -- it's basically the case with extort, and to a lesser extent exalted.

Or it could come with a mana cost limit, either fixed, or equal to the creature's CMC or something.