While Final Fantasy XIV hasn't yet been announced, its existence has effectively been confirmed. Right now it appears to be in the hypothetical stages, with many ideas still on the drawing board. Here's what we know.

 
The console Final Fantasy XIV appears on is still undecided, but it appears to be leaning towards cross-platform.

It's a given that PlayStation is the main choice, but Peter Moore of Microsoft said that they're always discussing with Square Enix whether or not to publish future Final Fantasy titles for both the Xbox and PlayStation. Since FFXIII is on both consoles, XIV will probably be too.

Hiromichi Tanaka of Square Enix has said that there's a higher chance of Final Fantasy XIV being cross-platform, but only from a technical point of view.

Shinji Hashimoto recently said that he has no ideas for the future (in terms of more Final Fantasy games for the Xbox 360), as they have their hands full now.

 
Yoshinori Kitase recently made a point about how all the numbered titles are very different to each other - that FFXIII, which has a sci-fi universe, changed drastically from FFXII. He concluded that there's no way of predicting what XIV will bring.

Obviously this means that the story, setting and characters have not been decided and could be anything... though based on his reasoning, XIV may stray away from the sci-fi-type universe in XIII.

 
Most staff are unknown, but Tetsuya Nomura will probably not direct Final Fantasy XIV. He has stated that he would like Final Fantasy Versus XIII to be the last numbered Final Fantasy he directs. Expect Yoshitaka Amano to draw a logo.

 
It's possible that the upcoming, untitled Final Fantasy MMO could end up as Final Fantasy XIV - the developers have stated that it could be a game in the FF series, a continuation, or a totally new game.

 
The release date isn't looking close, with Final Fantasy XIII having planned releases for 10 years. This might mean that Final Fantasy XIV will take a long time to be released, but developers have not said anything about this.