I've just read the Game Informer review of FFXIII-2 and they dropped a pretty major tidbit on the FFXIII-2 ending. They don't reveal the actual ending, of course; it's something else. Let me quote the article directly.

Quote Originally Posted by Game Informer
To make matters worse, you aren't even getting a complete story. The canonical ending (there are multiple other "what if" endings) concludes with the words "to be continued," and it's not a scenario likely to be resolved with future DLC. After the credits, I sat in disbelief, wondering if I had done something wrong or somehow missed the true ending. I didn't. Square Enix intentionally left the story incomplete. The announcement of Final Fantasy XIII-3 is inevitable.
What do you all think about this? I'm personally kind of miffed. First, I don't like the idea of playing a story without a satisfying conclusion. And I'm also probably not willing to go on another adventures with the characters from FFXIII. Two was pushing it as it is. Of all the games to make into a large franchise! Why does SE love FFXIII so much?