I think we've all got our own criteria on "what a Final Fantasy can be" that has evolved over the years. In my opinion though we need to allow ourselves to stay open minded about this stuff; as long as it works and it's a really good game, then should we call it something else just because the game mechanics have changed? Yes I would like to see the franchise return to its roots every now and then - the last time it really did this was FFIX which, as we all know, was the best in the franchise. But that also doesn't mean it's not OK to push at the perceived boundaries and try something different.
So maybe I'd like FFXVI to try something a bit more traditional; some kind of turn-based system and full control over the entire party. But I don't think that's all Final Fantasy can ever be. Mechanics are just mechanics, they're not the be all and end all of any game. What really matters, at least to me, is how the game makes me feel while playing it. And this is why I'll still defend FFXIII from a certain amount of its criticism, because for all its flaws (and there were many, let's be clear on that) it still felt like a Final Fantasy to me. I still got invested in the world, invested in the systems, invested in (at least a couple of) the characters and choked up a bit at the ending. To me, those feelings - the sum of all the game's parts, whatever they may be - are what I use to determine if something is a worthy Final Fantasy game, rather than a checklist of approved game mechanics.