Ron Weasley can be another main character. He provides the loyalty and friendship aspect. In a story about the mist flames, Ron could be the one who stands by Harry's side in a dangerous situation, like when they have to enter an area filled with the mist flames to retrieve a powerful magical artifact.
Often, Harry Potter himself is a main character in these fanfictions. Since he is the central figure in the original series, he is likely to be involved in any new development related to unique magic like sun flames. He might be the one to discover the power of sun flames and use it to protect Hogwarts or the entire wizarding world.
Harry Potter himself is often a main character. After all, it's a Harry Potter fanfiction. He is likely to be the one facing the challenges related to the mist.
Naruto himself is often involved. He might be intrigued by the power of the crimson flames and try to understand how it can be used for good. Sasuke could also be there, perhaps seeing the crimson flames as a power he could use to achieve his revenge or later, to protect the village. Sakura might be involved in healing those affected by the crimson flames' power, or she could be learning to fight alongside a character who wields this power.
Often, Naruto himself is involved. Since it's in the Naruto universe, he might be the one to encounter the Demon of the Mist and try to figure out what to do about it. Also, characters from the Mist Village like Zabuza or Kisame could be main characters as they have connections to the Mist and its secrets.
Harry Potter himself is often involved. He might be the one to encounter the first signs of inflation, whether it's a magically inflating potion or an enchanted object. His friends, like Ron and Hermione, are also likely to be part of the story, helping him figure out what's going on.
Ron Weasley could also be a key figure. He might be Harry's loyal friend as always, standing by his side during all the changes that Harry is bringing about in the wizarding world as a god. And there could be new characters introduced as well, perhaps other gods or magical beings that interact with Harry in this new fanfiction realm.
In some cases, Albus Dumbledore could play an important role. His wisdom and power in the wizarding world would make him a key figure in any situation where the two different universes collide. He might be the one to first discover Alexstrasza's presence and figure out how to make use of her abilities in the fight against the dark forces in the Harry Potter world.
Obviously, Harry Potter himself is always a main character. He is the one most directly associated with the concept of death in the original series, what with his parents' death and his multiple near - death experiences.
Ron Weasley could be another main character. He might be initially skeptical about the piano's magic but then be amazed when he sees the extraordinary things it can do. Maybe he accidentally discovers a new function of the piano while fooling around with it in the Gryffindor common room.
Harry Potter himself could be one of the main characters in such fanfiction. Maybe he gets an automail limb after a particularly dangerous encounter with Voldemort. Hermione might also be involved, using her vast knowledge to help understand and improve the automail technology within the wizarding context.