I think there will be meaningful things to do on the floating islands. I think that you will start on the floating islands after being recovered from the underground segments, and have to get abilities to be able to return to the floating islands, like the time travel ability. I also think the underground will be a home to many of the new dungeons.
But just imagine all the korok seeds (or their equivalent) on all those islands. Maybe even particular resources for various crafted items.
It really is wide open to speculation, which I know bothers many people. But I just love all the possibilities. Breath of the Wild over delivered on my expectations. I doubt this game will, but I can dream.