The only card game I'd play in FFXI would be Triple Triad. (can't stand Tetra Master)
I'd like the whole challenge random NPCs in towns feature from FF8.
Main storyline characters would grant some of the stronger cards.
There could be a method of taking Soul Plates and turning them into cards too

(Cards would have to be in a separate inventory)
Perhaps Cards could be transformed to and from Soul Plates, so it would become a new way to store soul plates.
Corsairs could potentially use these cards in new Job Abilities.
Puppetmasters could potentially use these cards as AI changers for their automatons. (new equip slot on Automaton)
Lots of potential here.