Geometer's Arthropod

Most card-selection engines pay you for casting any spell; this one specifically rewards a mana cost that scales. The clause narrows the payoff to a distinct family of cards (fireballs, X-draw spells, overloaded X-cost creatures and the like), and in doing so it ties the quality of the dig to the size of the spell: a small X shows you a couple of cards, a large X hands you real selection out of a deep look. That is a smarter piece of design than a flat "draw when you cast" trigger, because it makes the payoff self-balancing. You only get the reach when you have already committed the mana, and the spell that triggered it is already on its way; the impulse-style filtering is a rider on top, not the reason to cast. The body reinforces the intent: a 1/4 is built to survive, to sit on the board across multiple turns while the X-spells accumulate. It blocks early aggression, it does not race, and it asks the deck around it to lean into the specific shape it cares about. Green-blue is the natural home for that combination of ramp and card advantage, and the trigger reads as a deliberate reward for building toward the top end rather than a generic value stapled to a creature. The randomized bottoming is the honest cost: you see X cards but keep only one, and the rest scatter rather than stacking, so the selection is a snapshot, not a tutor.