Personally, I think the issue is that they pander to the fanbase too often. (Which they then receive backlash for)
Example! As was mentioned previously, open world is the way that many games are going nowadays and they tried to do that with FFXII. Personally, I really liked FFXII (Especially the opening sequence with Reks) but there was pretty massive fan backlash from the openness of it (Specifically the lack of direction), so SE recoiled and went in the complete opposite direction, which you see in FFXIII, which was met with even more backlash.
I think that it may be a bit too soon to say if Type-0, Verses XIII, or XIII-2 will get the formula right. Only time will tell~
However, I do think that the FF series is how to do sequels right. The battle system and game mechanics are improved on with every game instead of just leaving it the same forever. They may not always head in the 'right' direction. (Many people were upset by XIII's battle system for instance), but they take risks and experiment, which is something to be praised in this era of generic Triple-A games that don't deviate from the 'working' formula.
I forget if it was Type-0 or Verses XIII, but the producer said that it will feature the fastest ATB system ever. I'm excited~