What about a combination card? The next turn after the card is played you can combine say two cards . Since you have a reflect card that reflects physical damage how about one the can reflect a percentage of a curse or poison attack?
I have notice that you can have duplicates of a card. This can be annoying if you have two card that you really dont need at the moment, like two hafling spy cards. What is any card you have to purchase to unlock appear only once and add a revive card that allows you to select a card of your choice from the grave? Maybe even allow for the purchase of multiple cards after all the cards in a set are purchased. Also thought about special cards that are stackable, not like the modifier cards but a special attack cards that you need like three or four played before it does any damage and then have it do some wicked damage to pay off for the wait. Just a few ideas.
What if the combination card need two of the same cards
Arm and Arm, Leg and Leg, (from yu gi oh)
EDIT: i probably don't make sense. sorry