May be obvious, but I thought I'd add some other common features of card-based games. Each game system will likely require some subset of these features to be functional. My personal motivation is the miniatures game Malifaux that replaces die rolls with cards drawn from your own Fate Deck. You maintain a hand of cards, but also "duel" other players by making public flips from your deck, which can be augmented or supplemented with cards from your hands. Anyway, the complete list of card game features off the top of my head:
(1) a discard pile (that may be private, top-card public, or publicly browsable in its entirety),
(2) draw-to-hand vs flip for all players to see (needs to be a clear process so secret draws aren't mistakenly revealed, and supposedly public flips don't end up in your hand where it would be all to tempting to reveal a different card than what was drawn),
(3) finite deck sizes (currently you can keep drawing cards and it silently recycles numbers back in),
(4) secretly passing cards between players (the number of cards passed is usually public information), or openly passing cards between players (from your hand to their hand, with all players seeing the cards),
(5) retrieving cards from your discard pile either randomly or by browsing and selection,
(6) "stealing" cards from other players hands either randomly, or after privately viewing their hand,
(7) a "reveal from hand" that doesn't cause the card to become "played",
(8) putting cards into play where they remain in play vs playing a card which immediately goes to the top of the discard pile. This may take the form of the card becoming a player controlled token in roll20, allowing it to be positioned, rotated, or have markers and modifiers placed on it. It would also help to be able to send this "token" to one of the discard piles when required
(9) searching the remaining deck for a specific card and drawing it into your hand, optionally revealing it to other players first,
(10) shuffling only the remaining deck (often after the aforementioned "search") vs shuffling the discard pile back into the deck.
(11) each player having their own instance of a deck, instead of a single public deck from which all players may draw cards into their hands (as I believe it's currently implemented)
(12) a public discard pile from which players send cards from their hands, but that may be viewed and retrieved from later by other players
Malifaux would require only (1), (2), (3), and (11), I believe, which would make me happy! I hope some of these other mechanics help other systems as well though. Thanks for all the hard work so far!