Keeper of Progenitus
Mana doublers usually charge you for the privilege of building around them: a chunk of colorless in the cost, a tap to fire, a restriction to your own lands. This Elf Druid instead does the doubling passively, symmetrically, and only for three specific basic land types: Mountain, Forest, and Plains. The symmetry is the design tension at its core. Every player who taps one of those lands gets the extra mana, so the card doesn't ramp you so much as it ramps the table, which makes it a strange fit for any deck trying to win a mana race rather than open one. The land restriction is the whole point: it rewards a manabase that already lives in red, green, and white, and offers nothing to an opponent leaning on Islands or Swamps. Note also the precise wording: the doubled mana matches the type the land produced, so a tapped Forest yields two green, not two of anything. That keeps it from laundering colorless or off-color mana into whatever you please. The 1/3 body is the giveaway that this was never meant to attack; it is a battlefield-wide ritual engine in elf clothing, built for a deck big and greedy enough that doubling everyone's mana is a price worth paying because it spends its own share better than anyone else at the table can.
