I'm not an expert, but I thought the XSL-FO stuff (which fop seems to be a variant of?) didn't support pictures? A trading card game would probably need that.

For a trading card game, you'd need some checksum and ID build into the files. Comments are possible in pdf (and all image formats, afaik) and could store the checksum and id. Hmmm.. a web interface that took an uploaded image and gave verified owner and correctness? Fun project.

(-: And for economics -- a Magic The Gathering clone playable and storable over the net... what a neat way to steal children's money. Please remember me when you guys start hiring! :-)