As far as I know, there may not be a 'Never Ending Story 3' officially released. But the film industry is always evolving, and new projects can come up unexpectedly.
To write a never-ending ending, you could have the main conflict resolved but introduce new characters or subplots that suggest there's always more going on in the fictional world. Another approach is to end with a prophecy or a hint of a greater challenge on the horizon.