Some of these franchises are nearly (or) 20 years old at this point. Many retain the same core gameplay or concept. It is unlikely to change in the next 20 years unless the games are no longer profitable. In the case of the latter it would be more likely (and profitable) to retire the series or franchise rather than take them in directions against gameplay or concept.
So I would say they will not change much at all. New interfaces are very possibly the most dramatic change coming to existing franchises.
What made Mario 64 or Ocarnia of Time so revolutionary were the methodology for controlling those games using pre established gameplay and devices. One was still jumping about the field of play in Mario 64, but how one did so was a revolution of method.
The original team that made the very first Final Fantasy is actually gone. The creator of the series left Square a long time ago. It's about following the same formula though.
I mean, look at XV, it has totally different combat system but it seems to be bringing the open world and using vehicles to travel back. Something we haven't really seen since IX-X.
About the music, sure, Nobuo was amazing, he can never be replaced. I didn't like the XIII ost one bit, found it bland. XV's composer is Yoko Shimomura however and to me she is on the same level as Nobuo (she is responsible for the KH ost).
A lot of franchises will continue to stay par for the course. Nintendo is known for continually innovating their franchises and I am intrigued to see if they will do another Mario platformer using the SM64 formula again at some point with new ideas mixed in, like a hybrid of the side scrolling with the 3D type of previous games. I didn't mind Super Mario 3D World, but I still like the older style better. I thought Galaxy was probably their best stab at innovating the franchise again.
A lot of franchises will stay the same and some will change, most devs don't care for change because it means more work. In an era where budgets and profits mean everything, people shouldn't expect new things.
If they stay profitable, they'll stick around. If not, they'll stop being made. Whether the original dev's still around is pretty irrelevant, you see series continuing all the time while the original devs have already left.
As for a Mario FPS, considering Mario has already been to just about every other genre, who knows.
When a dev team gets entirely replaced or have changes in management, its not unlikely the direction changes even if the core idea might still be there.