Good game and great idea, for me I'd say the final touches to make it a great game are to make the enemy AI a bit dumber (right now it's a little tough with it always tracking the ball), giving it some abilities of it's own to fight back against the player, and a win/fail state that could perhaps lead into different game modes?
Also managed to get the following combo of cards:
- Get the double paddle card
- Get a couple cards to double your paddle size
- Bingo! You now cover the whole of your side of the screen and become unbeatable :D