A recurring phrase in Michael Ende's The Neverending Story, often seen when some character or characters drop out of the story, is ".. but that's another story and shall be told another time". I believe this is what's being referenced by Bastian, Atreyu, and Falkor near the very end:
"The Water asks you," Falkor translated, "whether you completed all the stories you began in Fantastica."
"No," said Bastian. "None of them really."
Falkor listed awhile. His face took on a worried look.
"In that case, it says, the white snake won't let you through. You must go back to Fantastica and finish them all."
"All the stories?" Bastian stammered. "Then I'll never be able to go back. Then it's all been for nothing."
Falkor listened eagerly.
"What does it say?" Bastian wanted to know.
"Hush!" said Falkor.
After a while he sighed and said: "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.
Given the whole theme of the book - sucking readers into the story in order to continue it themselves - I think these unfinished tales are a very important thing for readers to appreciate and make note of. For anyone who wants to do what Bastian did and continue telling the Neverending Story, these stories are an easy place to start: loose strands left over which any other storyteller can continue working with.
What are all the "other stories to be told at other times" in The Neverending Story?