Progenitor Mimic

Clone effects had been around for years before this, but they all shared the same shape: pay your mana, get one copy of one creature, and the transaction closes. This one rewrites the deal by stapling a recurring token-maker onto the copied body, so the value compounds every upkeep instead of resolving once. The trick that makes the engine matter lives in the conditional. Each token it creates is itself a copy, which means the token inherits that same upkeep trigger as part of the copiable values; but the trigger carries an "if this creature isn't a token" check, and a token fails that check every time. So the ability is present on the copies and simply never fires from them. That single clause is the throttle: without it, self-copying would spiral into an unbounded loop, and with it you get a clean one-copy-per-turn faucet that can never accelerate. That governor is what lets the design exist at all. The printed 0/0 Shapeshifter body is a non-entity; you never resolve this to be a Shapeshifter, you resolve it to become the best creature already on the battlefield and then keep duplicating that creature every turn after. Point it at a mana producer and you have a ramp engine; point it at a finisher and you have an army; point it at a strong enters-the-battlefield trigger and the upkeep token rings that bell again and again. Six mana, sorcery speed, and a do-nothing turn before the first token arrives are the honest half of the bargain.