As has been stated before in this thread it should at the very least have:

* Variable rule sets allowing players to play for cards/gil/items or just for fun.

* Some cards obtainable via quests, monster kills/drops or events.

* Some cards obtainable via NPC shop.

* Sizable card library with of course additions coming at later points.

At the very least this would create a good mini-game and a great distraction for players to participate in. Having more things to do in FFXIV is never a bad thing.