A potion of nullifying would have a similar effect to a bucket of milk. A player with the nullifying effect would not be able to have any new effects added to them. This could be useful for exploring an ocean monument with protection from mining fatigue, or exploring an end city without worrying about levitation.
A Potion of Nullifying would be brewed in a brewing stand by adding cheese to an awkward potion. Redstone dust, gunpowder and dragons breath could be added afterwards to alter the potion.
A Potion of Nullifying would clear all effects from the player upon drinking and prevent any effect except nullifying from being added while the effect is active. Both good and bad effects would be prevented, giving the potion potential for both offensive and defensive use in combat. The player could remove the Nullifying effect by drinking a bucket of milk.
Adding a fermented spider eye would convert the potion into a Potion of Amplifying.