[2019] If you could pick one card to add to the Battlegrounds pool to wildly change the meta, what card would it be and how much would it cost?

We playtested Sylvanas and Faceless and it was too busted!

We used to have a hero that made your left most minion poisonous (Putricide) so we passed over unstable ghoul. Now that he gives +10 attack (+20 in the future actually) we can include ghoul.

We are adding an existing Demon in December, bonus points if you can guess which one.