We have developed a backend that allows you to store your game data and progress via a unique ID that can be stored in a QR or NFC tag linked to a physical object, it works as a key to a could storage service. It is free to use, you can register in http://developers.twinsprite.com start creating identities to your objects and integrate it in your mobile game with the Android, iOS and Unity SDK.
A use case is a mobile RPG game with a player hero figure with a linked NFC tag, you can store all your player inventory and progress in the figure. If you scan the figure in a different device you can continue playing in the previous game point. Another use case is the multi-game support, continuing with the RPG example, imagine that the player profile data structure is shared between multiple games, for instance it follows the Dungeons & Dragons d20 specs, it allows you to share your level, items... between multiple games via scanning your player hero figure.
We are looking for people interested in use physical objects in his games and start using the Twinsprite SDK.
Even though i am not interested in trading cards game i see a huge potential here, you could store game data on a real object and then transfer in another part of the world where the other player could 3d print the object along with the game data , still a niche market , but makes sense to me, congrats for your bright idea