By which I mean, a series should always exhibit a clear lineage to its games, otherwise why even be a series. Sure, sometimes there might come a major paradigm shift point where the gameplay drastically changes, but then the series should continue with that new lineage, like Resident Evil for example. After Final Fantasy X, and all the perturbations at Square, there came Final Fantasy 12, which was nothing like the rest of the series. It was a great game so it was easier to let that slide and you'd think, a-ha, that's that paradigm shift of the series. But then comes Final Fantasy 13 and it's absolutely nothing like either FF12 or the rest of the series. Then comes Final Fantasy 15 and it's nothing like FF13 or the rest of the series. Then comes Final Fantasy 16 and it's nothing like FF15 or the rest of the series.
If any of these game were called something else entirely and didn't have the FF naming conventions, you would never think or guess "ah yes, this is a Final Fantasy game". When FF17 comes out, it's gonna be nothing like FF16 or the rest of the series. At this point they're just doing whatever and slapping the famous IP brand name onto it.

This concludes my Ted talk.