Beyond Final Fantasy XIII and XIII-2, the Fabula Nova Crystallis series extends to Final Fantasy Versus XIII and Final Fantasy Agito XIII (renamed Final Fantasy Type-0). Of the planned "trilogy" of games, the only one yet to see release is Final Fantasy Versus XIII. In fact, nothing has been heard of the title in quite some time – it was a no show both at last year's Tokyo Game Show and this year's E3 Expo. A demo of the next-gen technology supporting Versus, the Luminous Engine, was given at E3, but Versus wasn't part of the presentation.
Kotaku's reporting that the game may be in flux or cancelled, though Square Enix reps refused to comment on the status of the title. With any luck, we'll find out more this September.