Trading cards? I like the idea. Forgetting the rules of the system since Nostr can handle anything with the right coding… I would imagine that the cards would function like NFTs or badges. Possibly you could open “packs” by Nostr activity being tracked as a “proof of participation” like notes, comments, or likes.
As for a Nostr app for card games: I can’t code to save my life, but I do love game design and have already designed one card game to teach my kids math. Designing a bit more of an adult game for this ecosystem would be fun if someone wanted to help me with the app end.