My suggestion:
Variations on card discarding. As it stands the plugin only seems to allow for one type of discard; the player chooses what they discard (or if the discard is the size of their entire hand, it's handled for them automatically). What about randomly chosen discards? What about discarding cards from the top of the deck instead of the hand? What about 'discarding' to the top of the deck, instead of the discard pile?
Enemy attacks forcing the player to discard cards (their choice or random) too?
Edit: also, mouse support. I don't seem to be able to select different cards using my cursor, only confirm the current selection. This might impact mobile compatibility, too.