Eldrazi Mimic

A 2/1 that is really a blank waiting to be filled in: the body it shows up with matters less than the body that lands after it. The trigger copies the power and toughness of the next colorless creature to enter, and only until end of turn, so the whole design orbits a single attack step rather than a board it permanently holds. That ephemerality is what pays for the rate. A two-mana creature that swings as a 10/9 for one turn is not a standing threat; it is a tempo burst, and the deck has to be assembled so the burst arrives when it counts. The optional, until-end-of-turn nature of the copy is doing the balancing work: you can decline the change to keep the 2/1, and you can never lock in the larger size, so the Mimic is always borrowing, never owning. This is why small colorless bodies do nothing for it (matching a 1/1 only shrinks the clock); the card wants you to cast something big and have the Mimic come along as a second copy of that body for the swing. It points only at the most recent colorless creature to enter, so sequencing relative to combat is the entire puzzle: drop it early, then let each escalating colorless drop pull it up to the top of your curve, keeping the cheap creature attacking at the size of your most expensive one.