I remember thinking on MANY occasions when playing or killing time in both FFXI and FFXIV that SE should implement a mini-game for card battling (both humans and NPCs). It would be similar to FFVIII where everyone, including NPCs, has a deck and should you both choose to challenge each other, you can have a mini-game of card battling where the victor obtains cards/money/in-game bonuses/etc.

This would be a deck-building mini-game where you carry around your cards with you (though they wouldn't take up a slot(s) in inventory and would just be like minions/mounts where you just have them (though can lose them in battles) and could battle anyone, anywhere. You can make decks in the menu and could, should you choose, try to get all the cards, some being very rare (some can be bought, others need to be won). It also would require a UI, not just something like the Jeuno casino from FFXI which was all text based (and I don't mean a separate, out-of-game app like Tetra Master was)....