There is a strong belief going around based on a tease from Motomu Toriyama (now personally i hope there is a new final fantasy, i really enjoyed playing them and an even better one would be perfection), and all rumors point to a possible announcement being made on September 1.
On September 1st Square Enix will host the Final Fantasy XIII Lightning Saga New Developments Presentation at 11:30 local time. The whole crew of director and producers will be on hand to help, this sounds to us like a possible good setting for the announcement. We believe it might be the next chapter in Final Fantasy 13. Mostly like FF XIII-3 will be announced.
Source: member.square-enix