­Leonin Battlemage + Allow Custom Symbols

Uh, let's reroll that.
+ Set One Submissions

Well; now you're taking the piss.
+ {} Finally!

So. A kitty that makes things bigger and caps off an infinite spell combo. Um, well, it really likes it when you cast Giant Growth and friends, I guess. And... a variant of infect which is pretty poor; but hey, you probably want to block it because it has a Norrit ability if you leave it around.

So.. I could do a cat that acts on untap? A cat that throws around -1/-1 and untaps? An infect variant that deals damage in the form of +1/+1 counters? A cat that taps things down?

That last one is most classical I think; I rather fancy the -1/-1 throwing though. Which makes it black. Now, for its untap clause...
It kinda feels like it should be "When something dies" but that's stupidly repeatable; from the source material - whenever something untaps? Whee! A STUPID number of activations; but only every untap phase. No, that's too silly. (And unintuitively so.) Whenever something is exiled? That could be kinda flavourful. I like that. It's gonna be largely trinket text, but it could be a build-around.

And even if you can't make use of it, this is still a strictly better Prodigal Sorcerer due to the -1 power. Which might be enough complexity to push this rare - but I think the 'build around' potential makes it uncommon. Certainly not common.

Cost... Well, strictly better than, says more than 3. Not much better than, and critters have gotten better since.. so I'll go with CMC4.

Note that unlike Prodigal Pyromancer/Prodigal Sorcerer, this can't ping players, losing a lot of the benefit in leaving it untapped.

The ability to fire it off multiple times means that 3 mana is probably still right, but it's not completely clear cut.

How about "Whenever one or more cards are exiled from anywhere"? That lets it combo with the Voidwarp Eldrazi, as well as the Ingest Eldrazi they're based on (Fathom Feeder and so on).

I'm surprised neither of you mentioned Goblin Sharpshooter. I know this isn't that... but toss in a Planar Void or some such and you're most of the way there.

Well true; this is that; but -1 power as well. And not red; which gets to be better at killing stuff. Does seem like planar void is a good next part of the build-around combo :)

And; uh; why wouldn't this be 'from anywhere' already?

This says "a creature is exiled". Things on the battlefield are creatures; everywhere else they're "creature cards". This won't trigger when Tormod's Crypt or Castigate or Dissipate exile a creature card, as currently written.