It would be cool if they were actually very cheap (as item) and you can get them by playing 500 or so games. Then, each time you beat someone that has a card, you gain it in your collection (not as item but as a trophy). Then ppl would actually be proud of their collections and try to collect more. If their collection would be shown on their user page as an album or something...
Market for them would be made than(ppl would probably hire artists just to make them those cards with their toris that look nice etc).
If the collection would have a value depending on the player's belt you manage to beat (higher belt, more value) then ppl would go around hunting for other players that have cards to incorporate them into their collections thus having a bragging right when gaining more valuable collections... idk