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.
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.
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.
One interesting 'Harry Potter Vengeance Fanfiction' could be where Harry discovers a long - hidden plot against his parents and sets out for a very personal vengeance against those involved. He might use his skills and the knowledge he has gained over the years to outwit his enemies in a more ruthless way than in the original books.
Perhaps in some fanfictions, Harry seeks vengeance for the loss of his loved ones. He could start by targeting those who directly or indirectly caused their deaths. For example, if he discovers that certain Ministry officials were corrupt and enabled Voldemort's rise, he would take action against them. He might use his status as the Boy Who Lived to expose their wrongdoings and bring them to justice in his own way, which could involve some form of magical retribution.
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.
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.
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 plot could be the angel of vengeance being a long - lost magical being that has a connection to Harry's family. It might come to Hogwarts to right some ancient wrongs that have been plaguing the wizarding world for centuries.
Another character could be Draco Malfoy. There could be fanfiction where Draco, having been on the wrong side for a long time, now has a change of heart and wants vengeance against his own family or the Death Eaters who used him. He might use his knowledge of the Dark Arts that he learned from his family, but now turn it against them in a bid for redemption through vengeance.