A proposed buff.
Not for the sake of a buff, but because I think it would make sense semantically, and I think it would be a cool mechanic.
What if Poison Sensitivity debuff made enemies who are immune to Poison become susceptible to them for as long as the debuff is active. Sorta how Hex enables targeting of Mischief. I don't think it'll break the game, but I think it'll make for some interesting gameplay.