Make Mercury Treads Great again
This thing has been nerfed directly (less tenacity) and indirectly (way fewer long duration stuns/fears in the game, way more displacements).
So what do we do about that?
Give Mercs a passive that converts a single displacement effect into a stun of equal duration (doesn't move you anywhere) with a cooldown of whatever. Now it actually counters Riot's love of knockup spam.