In an interview with Yoshinori Kitase and Takeshi Arakawa, IGN asks if Final Fantasy will ever return to the old-school style. Kitase replies saying there is no way of predicting what Final Fantasy XIV and XV might bring:
When it comes to the Final Fantasy series, all the numbered titles are very different to each other. So FFXIII has no black warriors and mages - it's quite a sci-fi universe - in that sense there's quite a drastic change between XII and XIII. There's absolutely no way we can predict what XIV and XV might bring.

This is the first time Final Fantasy XIV has been mentioned properly by a Square Enix developer - not as a hypothetical title.
Source:
IGN (Martin Robinson; May 7, 2009)