I think Luna could be a very interesting main character for this. She has a unique perspective on magic and the wizarding world. Her ability to see things that others don't, like the Thestrals, might help her in the Horcrux hunt. She could also use her creativity and out - of - the - box thinking to outwit the dark magic protecting the Horcruxes.
Neville could also be an excellent choice. He has shown great courage throughout the series. His knowledge of Herbology could come in handy when dealing with Horcruxes that might be hidden in or protected by plants. And his growth from a shy boy to a brave young man would make for an interesting arc in a solo Horcrux hunt story.
The main characters are obviously Harry and Daphne. Harry, being the Boy Who Lived, has a lot of experience with fighting Voldemort and is determined to destroy all the Horcruxes. Daphne, on the other hand, is likely a Slytherin or has some connection to Slytherin. She brings her own set of skills and knowledge to the Horcrux hunt. Maybe she has a good understanding of dark magic or some family secrets that can be useful.
One idea could be that the character discovers a hidden Horcrux in a place no one expected, like in an old, abandoned wizarding prison. They have to face numerous dark magic traps alone. Another could be that they are being hunted themselves while on the Horcrux hunt, adding an extra layer of danger.
It could be a new character, an orphan who has just discovered they are a wizard. They come into the wizarding world all alone, not knowing anyone or anything about this new place. They stand alone because they don't have a family name or background to rely on like the Weasleys or the Malfoys.
If Harry were to create a Horcrux in fanfiction, it would likely lead to a great corruption of his soul. Horcruxes are dark magic, and the act goes against all that Harry stands for in the original series. He might start to show signs of increased aggression, a detachment from his friends, and a growing attraction to the Dark Arts.
Harry, Ron, and Hermione would be obvious choices as main characters. They already have a lot of experience with magical mysteries and adventures at Hogwarts. Their knowledge of spells and the castle's secrets would be very useful in a ghost hunt. Another option could be the Weasley twins. They are known for their mischievous nature and creativity, which could make for an interesting take on a ghost hunt. They might use their prank - related magic items to track or interact with ghosts in a unique way.
The Horcrux hunt with Harry, Fleur, and Tonks in fanfiction could be full of adventure. Harry has his own experience and determination. Fleur could add an element of mystery and allure. Tonks, with her unique magic, is quite a valuable asset. They could start by researching the possible locations of Horcruxes in old wizarding libraries. Fleur might be able to charm the librarians into giving them more access. Tonks could change her appearance to look like a different wizarding scholar. Harry would be constantly on the lookout for any signs related to Voldemort. Their journey would be filled with close calls and unexpected allies.
It could be Dumbledore. In this fanfiction, Dumbledore might see Harry making a Horcrux as a threat to the wizarding world and tries to stop him at all costs. He could be the main antagonist because he stands in the way of Harry's actions, even if Harry has what he believes are good reasons for making the Horcrux.
It could be a rogue wizard who has a vendetta against the Dursleys for some reason and decides to attack when Harry is home alone. Maybe the Dursleys once unknowingly wronged this wizard's family, and he wants revenge.
Harry Potter himself is often a central character. Since he was the one destined to defeat Voldemort and destroy the Horcruxes in the original story, many fanfictions continue his journey with Horcruxes in new and different ways. Hermione Granger also features prominently. Her intelligence would be crucial in understanding the nature of Horcruxes and finding ways to deal with them.
Obviously, Harry Potter is one of the main characters since he is the one who is a Horcrux. His friends Ron and Hermione are likely to be main characters as well, as they are always by Harry's side in the original story and would play important roles in dealing with this new situation.