Crackdown Construct
A combo engine hiding inside a 2/2 body. The growth trigger fires on activated abilities, not spells or attacks, which sends the deckbuilding brief in an unusual direction: you are not looking for a board to swing wide, you are looking for an artifact or creature ability you can fire over and over for free or nearly free. Pair it with a cost-free activation loop (an untapper, a zero-cost tap ability, anything you can crank without spending mana) and the +1/+1 stacks as many times as you have iterations, turning the construct into an arbitrarily large attacker in a single turn. The deliberate exclusion of mana abilities is what stops the whole thing from being trivial: it cannot feed on a Llanowar Elves tap or a Sol Ring crack, so the engine has to be assembled from genuine utility activations rather than your ramp. That restriction defines the puzzle. The body itself does nothing remarkable, which is the point; this is a payoff whose entire value is contingent on the engine built around it, a one-card win condition for a deck that has already solved how to activate something repeatedly. Outside that shell it is a 2/2 that occasionally gets a little bigger, which is exactly the trade the design demands.
