It would appear that they have several non-FF games in the works. Including another MMO, several Kingdom Hearts-related games, several Dragon Quest games, a Star Ocean game, and a third entry into the Parasite Eve series. The non-FF games they are working on far outweigh the FF games in the works.
As far as FFXIII goes, I don't know much about the story, but it would seem that they are separate entries dealing with similar mythologies. It doesn't sound like they require you to play all three in order to understand the storyline of one, but that you have a better grasp on the overall story if you play all three games.