Gruul Beastmaster

Riot's whole pitch is a fork between speed and size, and this is one of the few riot creatures whose second ability actively wants both halves of that choice at once. Take haste, and its attack trigger dumps its power onto another attacker the turn it arrives, turning a 2/2 into a repeatable pump before the opponent has a blocker plan. Take the counter instead, and it becomes a 3/3 whose trigger pushes +3/+0 somewhere it matters, growing more dangerous the longer it survives combat. Either way the payoff scales with its own power, so any outside buff double-dips: raise the Beastmaster and you raise whatever it points at. That feedback loop is the design idea here, a creature that rewards you for feeding it rather than protecting it, which is exactly the aggressive, board-committed posture a Gruul shell wants. Its ceiling is bounded by two constraints working in tandem: the trigger fires only on attack, and it feeds only a creature you already control. So it does nothing to break a stall and everything to snowball one you are already winning. Think of it as a distributor of pressure rather than a source of it: give it a full board and it turns a wide attack into a lethal one, but hand it an empty battlefield and the +X/+0 clause has nowhere to go.