It would be nice if the display of cards could be made conditional upon the state of other cards.
In some subjects - e.g. learning the vocabulary of a language - the knowledge required is "flat" and traditional flash cards are appropriate. However for others, such as Higher Mathematics, it only makes sense to ask about some topics if there is reasonable knowledge of others.
It seems to me that the best way to implement this is to have a "heirachical" card type that can be used to define interdependent several cards at once. Another alternative would be to have cards that can query the ID of other cards, but this looks like it may be a bit flaky.
I will have a look into coding this myself, but I would be interested in what others think.