It appears that the next PSOne Classic release from Square Enix after Final Fantasy IX will be Threads of Fate, a dual-perspective action RPG released originally in the US in the year of Patrick Bateman (2000 AD). No official release date is known.
According to a recent Joystiq post, Square Enix producer Shinji Hashimoto and the people who follow him, are reason we'll see this re-release. Hashimoto polled his dudes, asking what game they'd like to see. Threads of Fate apparently popped up enough to please Square's board. Next time, though, we hope fans will scream and rage for Another Mind. Granted, it never hit the US, and well, we have the slightest idea what it's about but oh my god look at that cover art.