Vesuvan Shapeshifter

Most copy effects commit once: you point at a creature, you become it, and the imprint holds until the copy dies. The wrinkle here is the recurring exit. The upkeep trigger lets you flip the creature face down for free at the start of each of your turns, and the morph cost of flips it face up again with a fresh choice of what to copy. That loop turns one body into a recyclable answer engine, re-aiming at whatever creature on the battlefield is most worth being this turn and abandoning that shape when the board changes. Crucially, the copy happens "as this creature is turned face up," so the famous interaction is not with enters-the-battlefield triggers but with "when turned face up" abilities: copy Brine Elemental on your upkeep, fire its tap-down clause, flip back down, and repeat, locking an opponent out of untapping turn after turn. The optionality cuts the other way against removal, too: the flip is instant-speed, so an attempt to kill the creature in its current form can be answered by turning face up and copying something with protection, a larger body, or simply a different vulnerability than the spell was pointed at. The printed 0/0 is academic; it never sits there, entering face down as a morph or already copying something. What it sells is optionality measured in upkeeps rather than a single declaration: a copy effect that treats its choice as a question you re-answer every turn, free to ask and cheap to change your mind.