Obviously Naruto himself and Queen Celestia as his mother. There could also be characters from the Naruto series like Sasuke and Sakura. They might come to the world where Queen Celestia rules to help Naruto or to explore. And from Queen Celestia's side, there could be her loyal subjects, like the ponies if it's related to the My Little Pony world where she is from. They could interact with Naruto in various ways, either being friendly or a bit suspicious of this new 'son' of their queen.
The plot might involve Naruto having to prove himself as the son of Queen Celestia. There could be those who doubt his lineage and he has to go on quests or battles to show that he is worthy. He could also learn about his mother's (Queen Celestia) history and the secrets of the land she rules, all while using his ninja skills in a new and magical context.
I'm not sure specifically as there could be many different interpretations in that fanfiction. It might be about Naruto having some special connection to a celestial being, like being the son of one. Maybe it involves Naruto having unique powers or a special destiny because of his celestial heritage.
Obviously Naruto would be one of the main characters since it's about him being the son of Celestia. There might also be other characters from the Naruto universe like Sasuke and Sakura who could be involved in the story, perhaps having different reactions to Naruto's new identity.
Well, it could be about a whole new take on Harry Potter's story where he is the son of Celestia. Maybe it involves him having some special powers or a different destiny related to his celestial parentage.
One storyline could be Naruto growing up in the Naruto world but having strange dreams and powers that he doesn't understand. As he gets older, he discovers his true heritage as the son of Queen Serenity. This leads to him having to travel to the Sailor Moon world to learn about his mother's legacy and gain control over his new - found powers.
Maybe the story focuses on Naruto's search for his vampire heritage. He could be on a journey to find out more about his mother, the queen vampire. Along the way, he meets other supernatural beings who either help or hinder him. His friends from the ninja world, like Sasuke and Sakura, might initially be in the dark about his true nature but gradually come to understand and support him.
Obviously, Harry Potter would be one of the main characters. And since he's the son of Celestia, Celestia herself might be an important figure, either directly or through her influence. Then there would probably still be characters like Hermione and Ron, as they are Harry's friends in the original story and would likely be involved in his new adventures too.
Naruto would likely be more refined in his manners. Since he's the son of a queen, he'd probably grow up with a certain level of etiquette. But he'd still keep his core values of never giving up and protecting his friends.
Obviously, there would be the queen vampire, his mother. She could be a powerful and mysterious figure, perhaps with her own motives and plans for Naruto. There might also be some vampire elders or nobles who play a role in Naruto's life, either guiding him or trying to use him for their own purposes.