Reflecting Pool

The chicken-and-egg fixer: it produces nothing on its own and everything once you have something else producing first. The design conceit is parasitic by intent: it reads the mana types your other lands can already make and copies one of them, which means a Reflecting Pool drawn alone produces no mana at all (with no other land to mirror, the ability has nothing to add), while a Reflecting Pool on a developed five-color board is the most flexible source you control. That dependency is the entire balance lever. Compare it to the dual lands or shocklands that fix two colors unconditionally; this one fixes every color you are already capable of making, scaling its quality directly to the rest of your manabase rather than fixing it on its own terms. The wording matters more than it looks: "any type that a land you control could produce" keys off potential, not what those lands have actually tapped for, so a creature-land or a land with an unactivated mana ability still counts as long as it could make that type. It rewards a board built wide on colors and punishes a hand that opens on it alone, a piece of self-correcting design that has kept the card relevant across decades of greedier and greedier manabases without ever needing a power-level adjustment.