If you were to make FF15

[Fynn]

-Fantasy setting, as opposed to a modern/cyberpunk setting. This is Final Fantasy. It should be fantasy.

Fantasy means "not real". Tons of futuristic technology that couldn't technically exist is "not real" enough to me. FF1 had robots and a rampaging mech in it.

FF has actually always blended fantasy and sci-fi. Like someone else has once stated - if you want a medieval-style RPG, play Dragon Quest.

Let's look deeper into that:
FFI - largely medieval, but with robots, flying fortresses and airships.
FFII - same here. The empire possesed the Dreadnought and later the party obtained an airship.
FFIII - I'm not quite sure about this, but it definitely had a lot of airships.
FFIV - airships, spaceships, high-tech towers - all this in a largely medieval world.
FFV - again, largely medieval setting, but we also have technologiacally advanced ruins and, of course, airships.
FFVI - this time we get a world that's sort of medieval, but there's a lot of steam engines. And let's not forget the airships.
FFVII - this time, it's the other way around - a futuristic setting, but with magic. Come to think of it, the smaller towns look medieval...
FFVIII - see FFVII.
FFIX - haven't played it, but I believe it's the medieval setting again - but with airships, of course. (And I believe the black mages are manufacted, right?)
FFX - the setting is kind of medieval but a bit more to the east. And of course Machina, airships, the Al Bhed Home, etc...
FFXI - I guess it's pretty much the same deal as with 1-5 and 9. Middle ages and airships.
FFXII - the setting here is strongly medieval but it also has A LOT of technology (including airships).

If FF was suddenly to become completely medieval, it wouldn't be the same. Just try to imagine a Final Fantasy without airships.