Renegade Doppelganger

Most copy effects ask you to choose your target on the way in: you cast the spell, you pick the creature, you become it. This one inverts the sequence. It sits on the battlefield as a 0/1 placeholder, doing nothing, until the next creature you play arrives and offers it a body. The trigger fires on entry, which means the copy lands at whatever speed your creature did, and the "until end of turn" duration turns each transformation into a one-shot lease rather than a permanent identity. The clause that strips the ability is the brake: one copy per turn, so it cannot ladder upward off multiple entries in a single combat. The design subtlety is worth stating precisely, because it cuts both ways. It copies the printed creature, so you capture the body, the keywords, the power and toughness of whatever just entered. What you do not get is a second enters-the-battlefield trigger: the Doppelganger was already in play when it changed, so it is not entering, and any creature whose value lives in its ETB gives you only its stats. That narrows the reward to creatures worth being rather than creatures worth replaying. The friction is plain: a turn where you play no other creature is a turn where this is a 0/1 that does nothing at all. It exists in superposition, waiting for your next play to tell it what it is.