Nanogene Conversion
Most copy effects run one way: you make a token of something powerful, or you turn a single creature into another. This turns the direction outward and points it at the whole table at once. Pick a creature you control and every other creature (yours and your opponents' alike) becomes a copy of it until end of turn, so the "copy" is not the payload; the mass conversion is. Aimed at your own board, it launders a swarm of small bodies into a squadron of your single scariest attacker, a board-wide anthem in disguise. Aimed with an eye toward the opponent, it can flatten a threatening enemy creature into a copy of something harmless, or strip a keyword-loaded blocker of everything that made it matter. The legendary clause is the necessary release valve: without stripping the supertype, cloning a legend across the battlefield would trigger the legend rule and force each player to keep only one legendary permanent with that name among those they control. Because it copies the chosen creature's copiable values rather than its current state, +1/+1 counters and auras on the chosen creature do not come along, which quietly caps how far the effect can push one overloaded threat. And the sorcery-speed restriction defines when it works: this is a proactive main-phase play built to be set up in advance, not an instant-speed answer to flash in over a wrath or a removal spell, so the reward for assembling the right board arrives on your turn, with the attack step still ahead of you.
