Kaza, Roil Chaser

The tension in this design is that the discount scales off Wizards while spending the Wizard doing the counting: tap her, and the next instant or sorcery this turn costs less, where X is how many Wizards you control as the ability resolves. That structure pushes the deck one direction. A wide Wizard board feeds a single large reduction rather than a trickle of value, so you want cheap Wizard bodies first, then a payoff spell worth the ramp. The haste is what makes it click: the tap comes online the turn she lands, no summoning-sickness delay before the engine turns on. The natural targets are spells that convert a big mana reduction into a swing (an X spell, an overloaded instant, a copy effect, a large draw), where knocking several mana off changes what you can do rather than merely saving a little. The cost of the power is the tap itself, which locks her out of blocking each turn she fuels a spell and leaves a 1/2 that any burn spell answers on the cheap; the engine lives entirely on a fragile body, and the discount is only as big as the Wizards you can keep alive around it. Izzet has long wanted a spells-matter payoff that also rewards a creature count, and this threads both by making the creatures the fuel for the spell rather than a distraction from it.