I think the next Final Fantasy XVI, which I'm sure has been in development for at least a few years at today, should mixed steampunk themes with modern themes.
I love both games like Final Fantasy XV, and also games like Final Fantasy VI and FFIX. And for this reason I think this could be an interesting way of being able to make the theme of a new Final Fantasy. Trying to combine the new Final Fantasy with the themes of the old Final Fantasy. And trying to mix a good dose of medieval environments with some futuristic and more modern elements.
(For example, regions of the world from the Middle Ages that use large carriages pulled by chocobo and giant salamanders, together with modern regions of the world, with locomotion machines, cars, seaplanes, trains etc ... And to all this adding also modern elements of nowadays, such as smartphones, drones [but more cool than real ones] tanks, mech monsters, three-headed dragons, Hydra, Cerberus, etc. ..)
Finding the right combination of these elements which, if properly mixed, can give life to a new Final Fantasy that recalls the classics very closely (FF V and VI) but which also manages to be more modern and contemporary, blending with elements of the modern Final Fantasy (FF XV and XIV).
And also creating a new battle system, action, dynamic, but also strategic. By combining a fluid action with actions that can be imparted in real time, parrying and dodging shots, changing jobs and using your skills wisely and thoughtfully. But with an adequate threshold of difficulty and not disproportionate or punitive even for the main plot, but manages to give challenges especially for the secondary missions.