So the game has been released in Japan, and only 4 days after the release date, a video of one of the 8 possible endings has been leaked onto a website.
I say again, this is an ending to the game, so major spoilers for anyone who wants to discover it when they play it.
On the Final Fantasy wiki, this ending is explained in detail, compared to the other 7 which are explained in one sentence, so speculation has transpired as to whether or not this is a canon ending, and that the FFXIII-3 domain is actually for another sequel game.
Source: http://zh-cn.justin.tv/archive/archive_popout?id=302683121 (contains the ending and game footage)
I say again, this is an ending to the game, so major spoilers for anyone who wants to discover it when they play it.
It shows Caius, fighting Noel and Serah, as the final boss in the game, weakened by the battle, he is seen knealt down in front of Noel, with part of his chest glowing a strange pink colour. After some conversation, he looks at Serah, and then goes to attack her with the Ragnarok, Noel defends her and in the final clash, proceeds to stab Caius in the chest, he stops himself at the last second, however Caius grabs Noel and forcefully stabs himself with Noel's weapon, releasing what appears to be a huge amount of Chaos from his body.
As Caius' body begins to fade away, Noel places the Ragnarok next to his lifeless body, and the sword disappears with Caius.
After some lengthy conversation between Noel and Serah, they travel through the time gate and land on a ship of some kind, as Cocoon falls a giant Sphere floats in the sky, which is where the Protagonists appeared from. Everything seems to wrap up, and Noel and Serah seem to be happy, but then all of a sudden, Serah's eyes show a strange light and Noel begins to shake her furiously, she loses consciousness and falls to the ground, and Noel holds her, screaming her name in despair.
Hope then appears from another ship and finds Noel and a lifeless Serah on the ground, and seems to look upset and distraught. They converse and then Mog also seems to die. Then Noel has a flashback of Caius and his glowing chest, looks at his hands in terror, and then the floating sphere in front of the ship starts to look menacing, as does the sky around it.
The scene then seems to move to Valhalla, where a petrified Lightning is sitting on Etro's throne, the words "to be continued" appear on the screen.
As Caius' body begins to fade away, Noel places the Ragnarok next to his lifeless body, and the sword disappears with Caius.
After some lengthy conversation between Noel and Serah, they travel through the time gate and land on a ship of some kind, as Cocoon falls a giant Sphere floats in the sky, which is where the Protagonists appeared from. Everything seems to wrap up, and Noel and Serah seem to be happy, but then all of a sudden, Serah's eyes show a strange light and Noel begins to shake her furiously, she loses consciousness and falls to the ground, and Noel holds her, screaming her name in despair.
Hope then appears from another ship and finds Noel and a lifeless Serah on the ground, and seems to look upset and distraught. They converse and then Mog also seems to die. Then Noel has a flashback of Caius and his glowing chest, looks at his hands in terror, and then the floating sphere in front of the ship starts to look menacing, as does the sky around it.
The scene then seems to move to Valhalla, where a petrified Lightning is sitting on Etro's throne, the words "to be continued" appear on the screen.
On the Final Fantasy wiki, this ending is explained in detail, compared to the other 7 which are explained in one sentence, so speculation has transpired as to whether or not this is a canon ending, and that the FFXIII-3 domain is actually for another sequel game.
Source: http://zh-cn.justin.tv/archive/archive_popout?id=302683121 (contains the ending and game footage)