I've often considered the thought of having a fantasy card available. I think a good approach would be taking a theme, not of instruments (ie: Session / Strings / Piano etc.) but of the nuance. An example being a night/dark SRX with a whole bunch of rich pads and keyboards - all of which have a 'darkness' to them. In the same respect one could be of light sounds... I know these aren't the best examples of what a nuance card could be - but It would be fun to have a card that doesn't classify itself by the instruments on it, but by how it can be used. (I realize that to a certain extent, this was done in the SRJV series with a few cards... as I've yet to try any of them, I posted this anyway).
Matt