I think a Triple Triad mini game would be a great idea, but it would take some re-design as well for the MMO factors.

In FF8, you play against NPCs/AI, so they're always responsive/willing to play. What would happen playing against other players someone took too long for their turn or forcibly disconnected in an attempt to not lose? Perhaps put a timer for each turn, and a default forfeit on disconnections?

In FF8, you can win other people's cards and lose your own. If we had this for FFXIV, would we get constant threads of people crying they lost their favourite/rare cards? Would the community be mature enough for something like that? Maybe you'd need to have it so the winner can get a copy of one of their opponents cards and neither person loses anything. (This would probably remove some of the thrill too)

Then there's how do you acquire cards outside of the game. Well we'd have lots of choices within FFXIV. You could have your retainer get you some randomly? you could craft some? Possibly get some as loot in dungeon/treasure map chests.

Finally, how would you do playing rules. In FF8, you had different rules in use on how to play depending on the region. (Random selection, combos, sudden death etc). Maybe you could have different rules apply depending whether or not you were in Thanalan, La Noscea or the Black Shroud?