The Prismatic Piper

Partner was built so two single-color legends could be stapled together into a two-color command zone, and it runs into an obvious gap: a player wants a Partner but has no second legend they actually care to run. This is the answer to that gap: a blank-slate second Partner whose only job is to widen the deck's color identity. Choosing its color before the game begins means it contributes exactly one color slot and a 3/3 without steering what the deck wants to do. That makes it structurally unusual among legendary creatures: it has no fixed identity, and it is designed not to. A Shapeshifter that picks its color is the tidiest possible rules expression of "I want the color access of a Partner without a second legend that matters," and the generic cost keeps that choice free of any pip that would prejudice the manabase. It reads closer to a deckbuilding permission than a card you cast for its own sake; the body that arrives is almost incidental to the color access it grants. Run alone as a colorless-until-chosen beater it does nothing of interest, which is why nobody builds it that way. It belongs beside a legend that carries the deck, doing the quiet structural work of being a color slot with legs.