It seems likely that another story in the Final Fantasy XIII universe is on its way. Motomu Toriyama, director of Final Fantasy XIII and XIII-2, recently made pointed statements about the open plot threads left at the end of the latest downloadable content, and promised that more answers are coming soon.
"The [FFXIII-2] Lightning download content had an ending that left a feeling mystery and hope," Toriyama told Famitsu (via Andriasang). "The day when the meaning of this will come to light is not too far off." His answer may have even included an unspoken time frame, since it was in response to a question about seeing more of the story at an upcoming anniversary event, to be held August 31 through September 2.
When Final Fantasy XIII-2 launched, I noted that the cliffhanger ending left the situation too dire to solve with downloadable content. Despite claims of other story avenues like novels in the works, it seemed likely that we would see a XIII-3 after all. Now that the final DLC has launched without any particular closure, Toriyama's comments seem to give a pretty clear signal of a future game.