I'm talking solely about the marketability of a medieval fantasy RPG.
And the counter-argument to that is that is despite it being a medieval fantasy RPG, Skyrim didn't sell primarily because it was a medieval fantasy RPG. It sold because it was open-world, immersive, and on PC had extensive modding legs. It also probably initially sold because it was an Elder Scrolls game, not something any other random medieval fantasy RPG game can automatically become.
It's the same way Witcher 3 isn't exactly selling because it's a medieval fantasy setting. It's selling because it's Witcher (which just so happens to be based on a book based on a fantasy setting)
Dragon's Dogma doesn't have the rabid, loyal fanbase it does just because it's a medieval fantasy RPG.
Kingdom of Amalur: Reckoning didn't sell just because it's a medieval fantasy RPG. (and witty ones might argue it didn't sell at all oh ho ho).
Heck, they're all good games in their own right but all added up to medieval fantasy RPG fatigue, just as there was WWII shooter fatigue, modern counter-terrorism shooter fatigue, and right now future war shooter fatigue.
If FFXVI is fantasy, you can bet it's going to be marketed on the shoulders of previous entries like FFXIV, FFXII, FFIX, FFIV, etc and not just because Skyrim sold well and is a medieval fantasy setting.
I think it's a little from column A and a little from column B, really. There's undoubtedly a very strong market for medieval fantasy RPGs - no doubt SE is looking at things like Skyrim, Dragon Age and the Witcher - and that's bound to factor into whatever decision SE makes. The adjustment in FF15's direction under Tabata and even some of the things about the shape and style of Lightning Returns make plain they are paying a lot of attention to Western studios - especially Bethesda and Bioware - closely. I imagine with W3's success they'll be analysing what makes that game and CD Projekt tick too. They've plainly realized Japan has had a rough time of it the past decade and are trying very hard indeed to learn, which is hugely commendable.
But beyond that, as well, it's actually been a long while since we've had this style of game from them in single-player anyway. FF15 has its realistic world with fantasy elements, FF13 has a sci-fi-meets-nature sort of thing going on with its split world. It's been a long time since FF12 and FF9. Square are also more acutely aware than ever that some people aren't happy with the realistic aspects of FF15 thanks to the streams, surveys and other feedback - Tabata himself has acknowledged this - and I'm sure the company at large will be keen to remind people that FF is broader than any one game.
Beyond that, I also think if you look at the pool of talent available around Square not on 15, you start to get a picture that those people tend to trend towards that style anyway. People like Ito (6, 9, 12 - though I'm beginning to suspect he's not involved), both Yoshidas (12, and 12/14 respectively) and such all seem to have leaned more towards that sort of style and setting. My creeping suspicion as far as FF16 goes is that they'll pair a known 'name' producer (N. Yoshida? Kitase? Ito?) with a relatively unknown director who has maybe worked on smaller titles - purely because of how successful their campaign of introducing Tabata as exciting fresh new blood has been. They'll want to repeat that, but if it's somebody without Tabata's handheld FF pedigree, they'll probably pair them with a safe pair of FF brand hands as well. There's a decent pool of talent who worked on their DS games, or their string of second-tier console games last gen like The Last Remnant, Nier, and so on, that could be drawn from there and potentially breathe a fresh new perspective into a new FF game. This'd be the first time that's happened on that scale in the series since FF7/8, where Sakaguchi began to step away, and that'd be significant.
Anyway... I'd put a fair bit of stock into the idea that whenever FF16 shows up if it isn't anything to do with Agni it'll have a FF12/14 sort of artistic look & setting. I personally would love them to make another whimsical game like 9 and take some chances on some not-uniform character design and such, but I don't think it'll happen in the main series for a long time, if ever again.