Just something that came into my mind while preparing pizza:

A ruleset and all the cards which will exist in the end have to be planned.
Every coin can by request be associated with a random card. That association is permanent, so when one of those coins is traded, the card is traded.

Further thoughts:
* Companies can build game interfaces and exchanges
* Blockchain can be forked at any time for addons, new versions, or complete new cardsets/rules
* What to do with never-redeemed coins or "lost" cards?

Going to eat now, would be glad to hear someone else's thoughts!