Deryn,
I LOVE the concepts, but the one thing that frustrates me about them (and the guardians, as much as I love them) - the use of gradients.
There's a filter out there that does some manual halftoning (like the woodcut style portraits on currency) that would make the design reproduce better on the cards and give it a more authentic look.
i can't remember the one i used to use, but it did a decent job. if anyone has it, try applying it and seeing what happens, I'm really curious how they would turn out...
it will sorta make the gradients do something like this....