A new character could be introduced. Maybe an old wizard who has been living in isolation for years and has a completely different view of magic compared to Hogwarts. This wizard could take Harry under his wing and teach him things like ancient spells and forgotten magical arts that are not taught at Hogwarts.
Ron could also be important. He might be torn between his loyalty to Hogwarts and his friendship with Harry. Eventually, he could choose to leave Hogwarts to be with Harry, bringing his Weasley family charm and his own brand of magic to the group. The Weasley twins might also get involved, providing some much - needed humor and their unique magical inventions.
One possibility could be that Harry discovers he has some other magical destiny outside of Hogwarts. Maybe he is taken in by a different group of wizards who teach him their own brand of magic in a hidden location. They might believe Hogwarts is too mainstream or that Harry has a special power that needs to be nurtured away from the school.
The main antagonists could be those who don't want Harry to receive a proper Hogwarts education. Maybe some dark wizards who fear that Harry will become too powerful if he goes through the Hogwarts training. So, they plot to keep him away from Hogwarts.
One possible plotline could be that Harry discovers a hidden magical world outside of Hogwarts. Maybe he stumbles upon a group of rogue wizards who teach him their own brand of magic, away from the traditional Hogwarts curriculum.
The Dumbledore character could still be important. He might be secretly watching over Harry, guiding him from a distance. Another character could be a new wizard or witch that Harry meets in the Muggle world who has a special mission related to Harry. For example, a young witch who has been exiled from the wizarding world but has the knowledge to help Harry understand his powers. And of course, Voldemort would still be a main antagonist as his threat looms over the entire wizarding world whether Harry is at Hogwarts or not.
Obviously, Harry is one of the main characters. And depending on how the fanfic is written, there could be some characters from the Forks world that are also main characters, like Bella or Edward if it's a crossover type of fanfic. There might also be some original characters created specifically for this fanfic to interact with Harry.
If the fanfic includes romance, then there could be a love interest for Harry. This character could be a fellow student at Beauxbatons. They might start off as friends but then develop deeper feelings for each other as the story progresses. Their relationship could be a central part of the fanfic, with all the typical ups and downs of young love in the wizarding world.
The main character is Harry. But in Pandora, he would meet a variety of characters. There could be a young Na'vi who becomes his guide, showing him around Pandora. Also, there might be a human scientist on Pandora who teams up with Harry. This scientist could be interested in the magical aspects Harry brings and together they could face the challenges in Pandora, both from the natural elements and the power - hungry humans.
It could be the descendants of the original Harry Potter characters. Maybe a son or daughter of Harry and Ginny, for example.
Obviously, Harry would be one of the main characters. He is the central figure in the Hogwarts world. Hermione and Ron could also play significant roles as they are Harry's closest friends and are often involved in his adventures.
There could also be new characters introduced in the fanfic. Maybe a young, talented wizard or witch who is just starting at Hogwarts and becomes involved in Harry's return journey. They could bring a fresh perspective and new abilities to the story. And of course, Dumbledore, even if he's not physically present all the time, his influence and legacy are likely to play a role, so he can be considered one of the important characters in a sense.