It's a variant on the level up mechanic that uses triggered abilities instead of mana payment to level up
Gogo, Master of Mimicry Final Fantasy #54 Legendary Creature Wizard , : Copy target activated or triggered ability you control X times
I could see it climbing to $5-$6 if it sees Standard play
At the very least, when your opponent is out of cards completely and isn't holding back any more cards, we can hold back on all our dead discard spells and then dump them to turn them into zombies
Credit: Wizards of the Coast A 4/6 with a kind of conditional war tax isn't horrible if you want to pillow fort, your opponents are generally going to need to pay 3+ mana to attack you, but it's not per creature, so if they need to they probably will
The mutate cost is high, but the payoff is more than worth it, especially since it's easy to obtain that much mana with traditional green ramp spells