During regular (table) play, I find the Mei character is too powerful. The ability to move diagonally is like being able to take two actions at once - "move" plus "climb" in one turn.
Silk might be odd to implement also, since one of his three abilities is chosen by the player (or make up your own??)
Twinzz would be good to implement - Two cards face up in a tunnel, rather than one face down.
Misty also seems powerful (not having played yet though) - ability to use the bullets in your hand as punches? What's the thematic connection (just curious)?