I like the card games from VIII (Triple Triad) and IX (Tetra Master). My friend and I once made versions of the IX ones, but it didn't work so well because the scoring system (letters and numbers) made no sense at all, so we had modified rules and brought in dice.

They need to bring those games back in for future FFs. Blitzball failed because it never really changed, and each game takes like an hour. A few minutes maximum, and with rare cards, and it'll hook me. Get something like that in XIII, and it could easily become my new favourite, those minigames made those PSX-era games great.