I emailed the guy that made FitDeck, Phil Black, suggesting the blank cards. He responded saying they've played around with that idea some, and plan to include them in the 2009 cards.
While not much use to me, I have all the cards I want right now (unless they come out with some "volume 2" decks), it'll at least be useful to other people who haven't gotten theirs yet, so they can wait and get the new run.
His email response:
Hi, Chris. Thanks for your suggestion. We have toyed with the idea of including a few extra "blank" cards for people to create their own favorite exercises. We plan to incorporate this concept in the next generation FitDecks which will launch in 2009. Thanks for the feedback.