Why was the Land of Telmar unpeopled when the pirates from our world were transported there?
How did Fair Olvin turn the Giant Pire into stone?
What happened at the Fight of Zalindreh?
The Chronicles of Narnia leaves stories untold. And not just side stories that one might create, like the fight between Corin and the Lapsed Bear of Stormness, or why is Ram the King of Archenland called Ram the Great, but stories where Lewis tells us that he is not telling us the story.
This of course leaves open the possibility that someone else can fill in the story. How would one approach telling such a story? You can just make up a story. The fight of Zalindreh (which is the same as the fight of Zulindreh?) does not necessarily have to have anything to do with Narnia and its future. It could just be a good story. But that wouldn't make it part of the Chronicles of Narnia.
To be a part of the Chronicles we would expect the story to have certain Narnian characteristics. For example, the story detailing the feat of Fair Olvin should reflect, like much of Narnian ethics and honor code, the knights of King Arthus, with Olvin playing the role of Jack the Giant Killer and the two-headed Pire modeled after the two-headed Thunderdell. Of course, we would expect Aslan to come in somewhere, perhaps to give Olvin the means of turning the giant into stone. And the story would have to manifest the themes of a Roman god reflecting an aspect of the true God. This would perhaps be the hardest part of composition. Manifesting the themes of Neptune as the god of the sea seems a bit out of theme for a fight in the desert between Archenland and Calormen. But maybe Uranus the father of the Titans and the primordial god of the sky or Vulcan the god of smiths and rocks.
Whatever the particular choice one makes the path is clear. To be part of the Chronicles of Narnia, a book has to respect the Chronicles of Narnia. The primordial stories must be kept intact, the story has to fit somewhere in Narnian history and have some impact upon it, and the writing, while no one can be Lewis, must try to accomplish what Lewis tried, bringing to light and teaching as if by osmosis, an aspect of the true God past the eyes of the watchful dragons.
If that is so for stories untold, how much more so for stories retold... but that is for another post.