There's a chance he meets some magical creatures that can talk in the American wilderness. For example, he might encounter a Sasquatch that has a deep understanding of the ancient magic in America. They could form an unlikely friendship and the Sasquatch could help Harry in his journey to adapt to the new magical environment.
Harry could meet students from Ilvermorny. They would have different personalities and magical abilities compared to Hogwarts students. He might also meet American magical creatures' caretakers who can introduce him to creatures like the Wampus cat or the Horned Serpent. Additionally, he could come across American Aurors who have their own methods of dealing with dark wizards.
In the fanfic, Harry Potter could meet the new generation of students. They would be in awe of him. He might also encounter some of the ghosts, like Nearly Headless Nick, who would surely have some interesting stories to share since Harry left. And there's a good chance he'll meet the caretaker, who would probably give him a hard time about coming back and causing a stir. Moreover, he could cross paths with some of the older students who idolize him and want to learn from his experiences.
I'm not sure specifically who 'harry potter king of fanfic' is. It could be a fanfiction author who is very well - known in the Harry Potter fanfic community for creating really great stories.
He might meet the house - elves that have been taking care of Potter Manor for generations. They could show him around the manor and tell him stories about his family.
He might meet Hermione. Since Hermione is always interested in academic things like Owls, she could be there to help Harry or just be involved in some research related to it.
One idea could be a fanfic where Harry Potter discovers a hidden portal in Hogwarts that leads him to the time of King Arthur. He could use his magic skills to assist Arthur in battles, perhaps using spells to defeat the enemies that Arthur's knights struggle with. For example, Harry could use the Stunning Spell to incapacitate a large group of invaders.
It could be about Harry Potter being seen as the rightful king in a fan - created story. Maybe it involves a scenario where he has a claim to some sort of throne within the wizarding world, perhaps in an alternate version of the events we know from the original books.
Obviously, Harry Potter is one of the main characters. Since it's about him being a king in some way. Hermione Granger and Ron Weasley are likely to be main characters as well, as they are Harry's closest friends and would play important roles in his new 'king' - like journey. There could also be other characters like Dumbledore, either in spirit or through memories, guiding Harry in his new role.