Each time a human comes to Fantastica, the whole world is essentially 'rebooted'. We see this in chapter 13, where Bastian and the Childlike Empress start off in total empty nothingness, before he begins to make wishes and the world begins to be (re)created around them.
On the other hand, some features of the world remain unchanged. Not only the Childlike Empress herself; also, for example, Atreyu and Falkor, or the country of the Sassafranians (mentioned in both halves of The Neverending Story, before and after Bastian's arrival).
This raises the question: was there really total nothingness before Bastian started to make wishes? Did Atreyu, Falkor, the Sassafranian country, and so on exist only because he wished for them? (If so, Atreyu has a remarkable amount of free agency for someone recreated only through Bastian's will.) Could anything he remembers from the previous Fantastica be recreated? (But that would include many things that had been swallowed by the Nothing.) In short:
How much of Fantastica survives between iterations - what and why?
Informed and supported speculation, based on the text or other good evidence, is acceptable.