As a really big fan of Magic: The gathering and Yugioh, I would appreciate some features that would allow for those kinds of games. I have a lot of cardgame-based elements in one of my campaigns, but unfortunately I can't host it here due to the lack of these features! I think these things would be pretty easy to implement based on what I've seen you already have, but if it's low priority I understand.
If there are ways to do these already that I have missed, I'll feel pretty silly, but I'm pretty sure there are not.
Essentials:
-Put a card on the field face-down. (You can place a card face-up on the field from the hand or deck, of course, but you have to place it before you can put it face-down. By then people will already know what it is.)
-Select a card that is on the board/in the hand and return it to the deck. (Instead of recalling all cards on the board.)
-Search the deck. (View all cards in a deck without drawing any; and pick one out, for deck-searching powers.)
Non-Essentials:
-Add many of the same card to a deck (Right now, if I wanted play, say, MtG, and i had 24 swamps in my deck, I'd have to upload 24 individual swamps. This is a massive pain in the patooty and it'd be much nicer to be able to just make 24 swamps appear with one upload.)
-Let players edit a deck (So that if they want to mess around with deck construction, they don't need me.)
-Ability to make 'piles' on the field, that function like mini-decks. (You can keep as many cards as you want in one spot, and anyone can look at all of them, and take one out, without cluttering the field with multiple card tokens.)