Square Enix have announced a direct sequel to Final Fantasy XIII due to be released next winter (2011)
The game is titled Final Fantasy XIII-2 and will continue the events that transpired in XIII.

“FINAL FANTASY XIII-2” is currently in production with an aim for producing high quality game content which exceeds “FINAL FANTASY XIII”

They also promise a new battle system and say that more details will be released in the future.

A trailer was also released, but if you’ve not seen the ending of XIII then be warned; this video will spoil it.