Storm King's Thunder
Where most spell-copying effects settle for doubling, this one scales: the generic X in the cost buys copies one for one, so the ceiling is bounded only by how much total mana you can pour into a single turn. That turns it into a delayed multiplier rather than a fixed one, priming your next instant or sorcery this turn to fork into a fan of copies with fresh targets. The tension is the two-step commitment. You spend the mana now on nothing but a promise, then have to follow with the payload spell in the same turn, which means the real cost is sequencing risk: you tap out (or close to it) for the setup, then still need enough left to cast the thing you actually want copied. That gap is where the card lives or dies. Point it at a big burn spell or a game-ending draw or damage effect and the copies compound the moment they resolve; point it at a two-mana filler because you had no better target and you have overpaid for a modest fork. The new-targets clause is what makes the copies more than raw redundancy: one removal spell becomes a sweep, one bounce spell clears a board's worth of blockers. It is a payoff card wearing the frame of a setup card, and it rewards holding it until the turn you already have a spell worth multiplying.
