Depending on how many cards are already ready, would it be possible to put out a number of cards for each challenge (maybe 3 or 4) and completing the challenge earns you a random one of those cards? After a while the cards could be reused so people can get the cards they missed, or get more of the same card for trading, or even have long running challenges set up for those individual cards. Just a few suggestions.
Also, are these cards being directed in general at long term members and players, or long term members and players with a certain level of playing skill? Just wondering how many people the cards are being aimed at