Avatar: The Last Airbender is indeed an original story. It has its own distinct style and narrative that sets it apart. The combination of elements like bending powers, diverse characters, and complex story arcs make it a one-of-a-kind creation.
Definitely not real. It's a fantasy world crafted with unique characters and magical elements that exist only in the realm of imagination and storytelling.
It's hard to say for sure without reading it, but likely Aang would still be a main character. He might be older and facing new challenges. Katara could also be a main character, perhaps with her waterbending skills being even more refined. Zuko too, as he continues to deal with the Fire Nation's future.