Give use Triple Triad (never could understand Tetra Master) and implement it into the actual game letting you be able to bet gil, maybe get rare item or have arena/tournament matches with no prize at all but just for fun (and the satisfaction of saying you won the "world championship" or w/e) you could build your deck through gameplay (normal enemies might drop cards oryoull find em in chests, harvesting (which wouldnt make sense) etc etc, rare cards might be found on BCNMs, NMs, raids etc etc.
Now THAT would be cool.