It's now a common practice for publishers to provide an update (ala Director's Cut, Special Edition, etc.) for popular titles, so it's no surprise that Kingdom Hearts II is next on the list.
According to a recent Jump Magazine article, Kingdom Hearts II: Final Mix is getting ready for release in the upcoming months. Unfortunately, it's initially only set for the Japanese PS2, as one of its major features is the English voices that Japan fans have (apparently) been clamoring for. However, there's another bit of bonus content for Final Mix, and it's a card battle system.
The GBA had Kingdom Hearts: Chain of Memories, which included a card battle game, and it's possible that Final Mix is recreating the card battle in special 3D form. We have nothing concrete at this point, though, so the Final Mix card system might be something deeper and more complex…
More details to follow.
Related Game(s): Kingdom Hearts II