Given the complete lack of closure at the end of Final Fantasy XIII-2, we became fairly convinced that another game is on the way. Square Enix is preparing to outline whatever will come next for the series, with an event planned for Tokyo on September 1, 2012.
A site from Square Enix teases that many key members of the Final Fantasy XIII development team, including the producer, director, and art director, will reveal "new information about the direction of Lightning's story" at a stage event in Tokyo on September 1. This echoes recent, extremely blatant hints from the game director.
As it happens, Square Enix is also holding an open fan event near PAX on the afternoon and evening of August 31 -- which is the morning of September 1 in Japan -- and it also promises new details about Lightning's story. The publisher seems to be aiming for a simultaneous reveal for both territories.
We can't say for certain if this means a new game, or some other form of fiction to flesh out the story. The team has hinted at novelizations as a possible avenue, but the company has been sitting on a Final Fantasy XIII-3 domain for almost a year. Don't be surprised if we see another full game in the series.