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 the remnants of the Death Eaters. Even after Voldemort's fall, some of them might still be causing trouble and Harry Potter has to bring them to justice for their past and present crimes.
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.
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.
In some cases, corrupt Ministry of Magic officials can be antagonists in 'Harry Potter Dark Business Fanfiction'. They could be involved in covering up or even profiting from the dark business. For example, they might turn a blind eye to the illegal trading of dangerous magical creatures in exchange for bribes. This would make them enemies of Harry Potter and those who strive for a just and clean wizarding world.
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.
Voldemort or his remaining Death Eaters are often common antagonists. Even after his defeat in the original story, in dark fanfiction, they could come back in some form to target Harry and Ginny. For example, a Death Eater who escaped justice might seek revenge on Harry by harming Ginny.
There could also be new dark wizards or witches who envy the power and fame that Harry and Hermione have. They might see the child as a way to get to Harry and Hermione, or they might simply want to prove themselves by challenging the family. For example, a dark wizard who has developed a new and dangerous form of dark magic and wants to test it on the child.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
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.
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.