It could be a dark wizard who discovers Hermione's hidden identity and wants to use it for his own evil purposes. Maybe he wants to harness the power that comes with her true identity to take over the wizarding world.
There might be a secret organization that has been monitoring Hermione because of her hidden identity. They believe her existence poses a threat to their long - held plans or secrets, and they will stop at nothing to keep her in check. They could be the main antagonist, constantly causing trouble for Hermione and her friends throughout the story.
One possible plot twist could be that Hermione is secretly related to a powerful and ancient wizarding family that has been in hiding. This would add a whole new layer of mystery and power to her character.
It could be a dark wizard who is jealous of Snape's relationship with Hermione. This antagonist might try to break them apart by causing trouble in Hogwarts or spreading false rumors about them.
Perhaps a powerful magical being from a different realm is the antagonist. This being might mistake Hermione for someone else or see her as a key to something in the wizarding world. So, it uses its magic to make her disappear. This being would be very difficult to defeat or negotiate with, making it a huge challenge for those trying to find Hermione.
Another possibility is a dark wizard or a group of dark wizards who are after some powerful magic that the Lestrange family has, and Hermione gets in their way as she tries to protect the family or uncover the secrets. They would be the main force that Hermione has to face and overcome throughout the fanfiction.
It could be the same group or person who tortured her in the first place. They might be trying to finish the job or silence her before she can expose their evil deeds.
Maybe it's a new dark wizard or witch who has infiltrated the Ministry. They manage to manipulate the situation so that Hermione is blamed for their own misdeeds and sent to Azkaban. Their goal could be to cause chaos in the wizarding world and Hermione was the only one who could stop them.
It could be an old muggle acquaintance who is jealous of Hermione's success in the wizarding world. Maybe they sense that she's different and are determined to find out her secrets and expose her.
It could be a new character introduced in the fanfiction. Maybe someone who has infiltrated the Order and wants to create chaos by turning them against Hermione. This person could be a spy from the Death Eaters or another dark - aligned group, with the goal of weakening the Order from within by making them lose one of their most intelligent and powerful members.
Voldemort himself could be the main antagonist in this scenario. He might offer Hermione something she desperately wants in exchange for her betrayal. Maybe he promises to spare her Muggle - born friends if she gives up the Order's plans and secrets. His powerful and intimidating presence could easily force Hermione into a difficult situation where she feels she has no choice but to betray.
It could be Bellatrix Lestrange. She's always been a cruel and powerful dark witch. In the fanfiction, she might have captured Hermione and bound her to the wall to torture her for information about Harry Potter or the Order of the Phoenix.