Viridian Joiner

Most mana-producing Elves tap for a fixed amount; this one ties its output to power, which turns a stat that usually only matters in combat into a mana multiplier. Out of the box it does nothing exceptional: a 1/2 that taps for a single green is strictly worse than a Llanowar Elves. The whole design is built around the assumption that the 1 will not stay a 1. Hang any cheap creature-buff on it (an equipment, a counter-doubler, a beefy Aura) and the mana scales with it, so a single enchantment can swing it from one green to three or four or beyond. That makes it a deckbuilding lever rather than a generic ramp body: it rewards the green decks that were already growing their creatures, converting toughness-irrelevant pump into a burst of ritual-like acceleration. The catch is the same one every power-scaling mana creature carries: the buff and the Joiner are two cards doing one card's job, and removing either collapses the engine back to a 1/2 that taps for one. It sits in the lineage of green's "mana from somewhere unusual" experiments, where the reward for jumping through a setup hoop is acceleration that ordinary one-drop dorks cannot match.