It feels like final fantasy is making a rather formulaic progression.
FF1 was huge, and FF2 and FF3 are pretty popular as well. But the next game I hear pracically everyone talk about is FF4. FF5 and FF6 were basically the same as 2 and 3, but then the next huge game was FF7. FF8 and FF9 were also popular, but not nearly as much as 7. Then FF10 came out, and it is wildly popular.
See the progression thus far? Every 3rd game hypes the series back up. Now, after 10, it gets a little wonky because 11 and 14 are MMOs. Frankly I don't think we should count them. I've heard people bash 12 and 13 constantly, saying they aren't nearly as good. Frankly, following the progression, it looks like these two are the 'filler' games, and the next one in the actual numeric series (FF15) might be the next great game people are looking for.
This is all just a theory from my own observations. 2, 3, not nearly as popular as 1. 5, 6, not nearly as popular as 4. 8, 9, not nearly as popular as 7. 12, 13, not nearly as well recieved as 10. 11 and 14 are MMOs and IMO not part of the numeric system, so maybe 15 might be better. Maybe.