Anointed Peacekeeper

Meddling Mage banned rather than taxed: it named a card and stopped it from being cast at all, a hard lock that lived or died on hitting the right name. This design trades that lock for a surcharge, and in doing so becomes a much more forgiving piece of interaction. The peek at an opponent's hand before naming turns what used to be a read into information, so you are no longer guessing at the format's boogeyman; you are looking at the specific threat in front of you and pricing it out of the turn. That the tax also lands on activated abilities of matching sources (mana abilities excepted) quietly widens the net past spells to planeswalker loyalty, equipment, and creature abilities, coverage a pure counter-on-a-stick never had. And unlike a Mage, it stays relevant after the naming resolves: a vigilant 3/3 attacks and holds the fort at once, so the card keeps earning its slot even against an opponent who runs four copies and simply eats the tax. The whole thing is an argument for soft locks over hard ones, the same instinct behind every effect that asks an opponent to pay rather than telling them no. A tax leaks where a prohibition breaks; this design bets that a leak you can plan around beats a wall that only works when your guess lands.