As some of you know, I am one of the authors of FINAL FANTASY XIII: LEGACY OF LINDZEI and XENOLITH, two fan sequels to FINAL FANTASY XIII. Information can be found here:
http://lindzei.dreamwidth.org/
Today, LIGHTNING RETURNS was announced:
http://www.ign.com/articles/2012/09/01/lightning-returns-final-fantasy-xiii-announced
According to this article, LIGHTNING RETURNS shares an extremely unusual amount of similarities with LEGACY OF LINDZEI and XENOLITH:
LIGHTNING RETURNS is "set in a world called Novus Partus". In LEGACY OF LINDZEI, the name of the planet is called "Novus".
"According to Square-Enix, the world will end after 13 days, and a massive clock will count down constantly in the corner of the screen to indicate this looming deadline." In XENOLITH, it is established the world will end in a 16 years, and a clock counts down constantly throughout the story.
Most unusual, and perhaps telling, is the following image, a map of Novus Partus:
http://oyster.ignimgs.com/wordpress/stg.ign.com/2012/08/lr-novus-partus.jpg
Now look at this. This is a map of LEGACY OF LINDZEI'S Novus, established in 2010 shortly after the release of FINAL FANTASY XIII:
http://i.imgur.com/ewqZH.jpg
I will spare you the details (all of them can be found at the LEGACY OF LINDZEI website linked above), but every single part of the Novus Partus map is a correct cardinal representation of the cities/locations listed in Novus: Castle Orbis to the south; the desert to its north; the body of water that separates it from Tenebrae; and Argo and Ezra, side by side, to the east. Even the sky in the Novus Partus image depicts Gran Pulse, as was established in LEGACY OF LINDZEI.
TL;DR: There is a REALLY good chance that Square-Enix copied multiple ideas from my FF13 fanfiction.