Sequels are a rare occurrence in the Final Fantasy series. Square Enix has only let it happen a few times in the series' history, so the announcement of Final Fantasy XIII-2 came as a surprise. Now as word of XIII-2's ending hits, the renewed possibility of a XIII-3 is circulating.
Intentionally vague spoilers follow.
The driving force behind the conflict of Final Fantasy XIII-2 is Serah's search for her sister, Lightning, with the help of new character Noel. Without revealing anything regarding that particular plot thread, the central conflict does take a turn for the worse and ends with the words, "to be continued."
We know that Square Enix is planning extensive episodic downloadable content, including chapters for characters like Lightning. Nonetheless, Square Enix did register a domain name for Final Fantasy XIII-3. The company has commented on shorter turnarounds for new games, so if we do see a full sequel it probably isn't far off.
Leaving hints for a sequel is nothing new in video games. Some of the more successful franchises of the past few years, like Mass Effect and Assassin's Creed, have capitalized on it. Still, it's a fairly recent trend for Final Fantasy games to have any sequels, much less multiple ones. The perpetually delayed Final Fantasy Versus XIII and Final Fantasy Type-0 take place in the same universe, as well, but aren't directly related to the plot.
Having finished the game, I would be surprised if Square Enix wraps up the plot with downloadable content. The problems presented by the cliffhanger ending seem too grave to solve without another full game. Square Enix's DLC plans instead seem focused on character-specific episodes, like adventures with Lightning and Sazh. Nothing is certain, but don't be too surprised if the company announces Final Fantasy XIII-3 in the near future. For more thoughts on the story and the game in general, check out our review.
Shacknews has contacted Square Enix, and will update as more information becomes available.