Great Train Heist

The extra-combat-step effect used to arrive in fixed, front-loaded packages: Aggravated Assault at four mana with a repeatable kicker, Relentless Assault at four for a one-shot, Seize the Day flashing back for six. Spree repackages that payoff as an à la carte instant, and the pricing hierarchy tells you where the danger lives: untapping your team and buying a second combat phase costs the most, and it is the mode that ends games. All three options push toward the same swing, which is what gives a modal instant this much coherence. The +1/+0-and-first-strike line turns a stalled attack lethal and rescues a bad block; the Treasure clause converts connecting damage into ramp for whatever the next turn holds. You buy exactly the pieces the board demands, and since the base card asks only a single red mana, there is always an affordable mode even when your resources are thin.

The subtle piece is the conditional on the extra combat: the untap-and-swing-again mode only grants a follow-up phase "if it's your combat phase," which pins the ceiling to your own turn and forecloses any end-step or instant-speed abuse of the sequencing. That clause keeps the effect at an instant's flexibility instead of being priced back up to four-mana sorcery territory. Stack every cost for a full alpha-strike finisher, or buy a single one for a cheap trick that still bends a combat step in your favor.