It is clear that not only do you need to delve deeper into your world, but you have to create a new world as well. You need fresh territory. Your characters need to roam further and further from the familiar.
You need to shake it up.
But by the end of Book#2, most of the assumptions I set-up in Book#1 are proven wrong. Characters get smarter. The nature of the universe gets more surprising.
So my plan when writing the Sky Island was 1) Deeper into the Old World, 2) Discover a New World, and 3) Change Everything. I think it worked. The magic and eye-opening revelation is still there.
Of course, I'm the author. We'll see on January 1, the release date, what my readers think!