The creators of Final Fantasy XIII have been notoriously tight-lipped about the Fabula Nova Crystallis trilogy, a series comprised of FFXIII, Final Fantasy Versus XIII and the PSP-exclusive Final Fantasy Agito XIII. During a GDC panel titled "The Crystal Mythos," director Motomu Toriyama gave a few scant details about the other two branches of the trilogy, and the few similarities connecting the entries in the franchise's 13th series.
Toriyama's main point during his presentation is that Agito and Versus won't be sequels to the recently released Final Fantasy XIII -- they'll both have their own unique set of characters, gameplay systems and overarching stories. The only thing connecting them is the "Crystal Mythos" motif -- a theme where the fates of ordinary humans are controlled by omnipotent gods. Players of Final Fantasy XIII should already be acquainted with this storytelling device.
The other detail about the series Toriyama mentioned was that, like XIII, the rest of the series will continue to focus on story-driven gameplay rather than allowing the player to explore an open world. For better or worse, it sounds like Versus and Agito will possess the same linearity for which XIII was frequently criticized.