I want a game that starts out Medieval but turns out to be inside a giant starship.
I played a game like that once. It starts out with your average medieval fantasy setting, but at the end you're fighting inside a spaceship using blasters. I forgot what it was called, but it was weird.
You forgot the in my opinion part. Xenogears destroy every single FF IMO. So futuristic can work with the right person. Medieval is just a meh setting to me.
Well, it's obvious what I think in what hands FF is currently. Better to keep it simple and "cliche" even for the story and setting, than another FF13 to happen with some pretentious deus ex machina writing.
Plus, a futuristic setting after FF13 would be too much. It just would be.
Xenogears is a different thing, and it's not a FF. Xenogears had Takahashi working on it, and considering how he did Xenosaga and Xenoblade, I think it's fair to say that the guy knows what he's doing when it comes to sci-fi.