Do you expect Final Fantasy 16 to be similar to Final Fantasy 15 in general ?
I don't know what they could do for the next game to make the worldbuilding really fresh, maybe create a world that's more bizarre/other-dimensional and alien in nature as opposed to something based on realism, fantasy or sci-fi alone. Maybe a world that takes inspiration from Planescape, End of Eternity or Gravity Rush?
We've already had so many flavors of fantasy, I personally would've liked something more eastern like the Type-0 sequel but then XIV has already kind of done that with Stormblood now, and so many games are capitalizing on the feudal Japan setting too, with Tsushima and Nioh 2 on the horizon. XII was inspired by the middle-east, VII was cyberpunk, XIII was more sci-fi. They could be really brave and make XVI a space opera but it'd probably stop feeling like FF and more like Phantasy Star/Rogue Galaxy instead. Maybe post-apocalypse? Mad Moogle Fury Road? Or a really gothic setting like Souls?
You have to be careful to walk the line between something that breathes new life into the series and something that doesn't feel like Final Fantasy at all, but it'd be easy to mess up and I don't know it Square is going to take that risk. In terms of gameplay, I'd be happy with an action-RPG if it plays like Kingdom Hearts, as Versus XIII was originally. Or just go back to turn-based, I'd be fine with something more dynamic like Persona 5's combat.
I kind of feel that they will go with this 'modern' stuff again. But I have no idea who the protag will be. Maybe girl again?
I think XIV will be going strong for years, but since it's so heavily rooted in the medieval fantasy side of things, I don't think bringing the mainline games back a "traditional" fantasy setting is right call unless they base it on something other than medieval europe again. I guess they could go gritty and do something like Vagrant Story, but I don't think they'd want that in a mainline title, we know Versus XIII was made more audience-friendly when it became XV.
And a female lead would be fine, but I think the next best decision is to have a cast with multiple protagonists/ensemble like FFVI. Maybe choose between 2 characters? Not an Octopath sized cast but something a little different like what Tales of Xillia did where you choose a male or female protagonist and get slightly different stories and perspectives, like starting the game in a different town.