Have all those loose ends from the Neverending Story ever been tied up?

Question

The most oft-repeated phrase in Michael Ende's The Neverending Story is:

but that's another story and shall be told another time.

Accompanied by this phrase, many minor characters fade out of the main storyline and aren't mentioned again, even though they do have interesting tales of their own afterwards which we simply don't see in the book. I made a list of all the instances when this happens:

• the four messengers (Chapter 1)
• the centaur Cairon (Chapter 2)
• the gnome Engywook (Chapter 7)
• the lion Grograman (Chapter 15)
• Hero Hynreck, Smerg, and Oglamar (Chapter 17)
• the mule Yikka (Chapter 21)
• Ushtu, Shirkry, and Yisipu (Chapter 21)
• the sword Sikanda (Chapter 23)
• Hykrion, Hysbald, and Hydorn (Chapter 24)
• Bastian himself (Chapter 26)

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This theme in the book is even alluded to in-universe, in the final conversation between Bastian, Atreyu, and Falkor when the latter two set off to tie up all these loose ends.

"The Water asks you," Falkor translated, "whether you completed all the stories you began in Fantastica."
"No," said Bastian. "None of them really."

[...]

"It says there's no help for it unless someone promises to do it in your place. But no one can do that."
"I can! I will!" said Atreyu.

[...]

"Falkor," said Bastian. "How will you and Atreyu finish the stories I have left behind?"
The white dragon winked one of his ruby-red eyes and replied, "With luck, my boy! With luck!"

_{-- The Neverending Story, Chapter XXVI}

So in-universe, all these stories will one day be finished.

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My question is about out of universe.

• Did Michael Ende ever write supplementary material set in Fantastica to tie up any of these loose ends, or even express an intention to do so (in which case there might be some relevant information among his notes)?
• If not, has anyone else made a concerted effort to finish off all these stories, whether in a way licensed by Ende or not?

Answer 1

Wikipedia has this to say...

From 2003 through 2004, the German publishing house AVAinternational published six novels in a series called Legends of Fantastica:

1. Der König der Narren [The King of Fools]. Kinkel, Tanja (2003).
2. Die Seele der Nacht [The Soul of the Night]. Schweikert, Ulrike (2003).
3. Die geheime Bibliothek des Thaddäus Tillmann Trutz [The Secret Library of Thaddaeus Tillman Trutz]. Isau, Ralf (2003).
4. Die Verschwörung der Engel [The Angels' Plot]. Fleischhauer, Wolfram (2004).
5. Die Stadt der vergessenen Träume [The City of Forgotten Dreams]. Freund, Peter (2004).
6. Die Herrin der Wörter [Empress of the Words]. Dempf, Peter (2004).

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Full disclosure, I have not read those works myself and do not know if they related to the specific characters you ask about.

Looking through Ende's bibliography, it does not appear that he wrote any other stories set in Fantastica. Some of the works remain in their original German.