Luna Lovegood might also be an interesting choice. Imagine if something really bad happened to her father or her friends in a fanfiction. Luna, who is often seen as a bit of an outsider but also very strong - willed, could be driven by a spirit of vengeance. She might use her unique perspective and knowledge of the wizarding world's oddities to seek retribution in a very Luna - like way.
Perhaps it's a magical creature that has been overlooked and abused by wizards for centuries. This creature, with new - found powers, decides to become the angel of vengeance. For example, a house - elf that has seen too much cruelty and now has the power to punish those who have mistreated its kind.
It could be a new dark wizard or witch who has a personal vendetta against Harry. Maybe they blame Harry for the downfall of their family or their own misfortunes. So they start causing trouble in the wizarding world, and Harry, in order to protect his friends and the world he loves, decides to take vengeance on this new threat.
One interesting concept could be the idea of a character seeking vengeance for a wrong done to them within the magical world of Harry Potter. For example, a character whose family was killed by Death Eaters might take on a spirit of vengeance. This could lead to them going against the normal rules and ethics of the wizarding world in their pursuit of justice.
One possible plot could be that Harry discovers that a group of dark wizards were behind the death of his parents in a more elaborate and concealed way than he initially thought. So he sets out on a journey to find and destroy them one by one, using his advanced magical skills and the help of his closest friends.
The Death Eaters are often common antagonists. Since they were the main followers of Voldemort, and Harry has every reason to be vengeful towards them for all the harm they caused. They killed many innocent people and were a constant threat to the wizarding world.
It could be a spirit named Orion. Orion could be a powerful celestial spirit that has been watching over Harry since he was a baby. He could be the one to finally reveal to Harry his true nature as a being connected to the celestial spirits.
Another possible candidate could be Tom Riddle before he fully became Voldemort. He was already exploring the darkest corners of magic at a young age. The idea of the Sith and their power might attract him. He could see the Sith as a means to an end in his quest for immortality and ultimate power. As a Sith apprentice, he could combine the knowledge of the Dark Side of the Force with his own wizarding knowledge, creating a truly formidable and terrifying entity.
Neville Longbottom could be a likely candidate. He has shown great courage and has been through a lot. If the situation was dire enough, Harry might train him to be a more effective fighter, bordering on a killer, to protect Hogwarts and the wizarding world.
It could be a jealous student. Maybe someone who has a crush on the same person Harry is attracted to in the story. They see Harry as a rival and figure that by poisoning him with a love potion, they can make him look foolish or get him into trouble.
It could be a former Auror who has turned to more shadowy ways of dealing with threats. After seeing the inefficiency of the Ministry's methods in some cases, they decide to take matters into their own hands and become a sort of assassin to target dark wizards.