[Haunted Evil Pajamas]

I would be all for helping in creating a TGC in the world of Sigrogana.  It doesn't seem like a big thing but those little minigames really do add a huge amount of personality to the world.

This is a bit of a tangent, but an amazing example to take from would be Spycards from Bug Fables, the way you obtain cards it by essentially scanning enemies in battle to learn some cool tidbits as well as their stats, then you get their card which you can form in a deck to play Spycards in.  The card game itself is incredibly simple (It's essentially a game of war but with stacking and synergy bonuses) but from the variety of cards there are makes you think, "It's amazing how much depth is in this optional side game."

I would also maybe take a note from Final Fantasy XIV and allow you to challenge some NPCs to this TCG as well, maybe have an ante system where players have to give one of their cards to their opponent if they lost?

(You can tell I adore stuff like this and would not hesitate to draw cartoony card designs)