Like, are you making your own cards or something? What about a 1/1 minion that costs 1, with the text saying "at the start of your turn, destroy the enemy if he has no minions"
MY X/Y POKEMON FRIEND CODE: 1418-7279-9541 In Game Name: Michael__
It costs 10 mana and its 1/1. You can define 3 of your most hated decks and if you meet one of those you get some insane battlecry...not game winning but crazy
If you meet another deck then those 3, it gets a negative battlecry and while you have it in your hand your cards cost plus 2 mana...