It could be a power - hungry student who wants to undermine Hermione and Favo Prefect. This student might be jealous of their status as prefects and the respect they get from teachers and students alike. So, they try to create problems for them, like spreading false rumors or sabotaging their projects.
Perhaps an old - fashioned teacher who doesn't like the new - fangled ideas that Hermione and Favo Prefect bring to the table. This teacher might be stuck in the old ways of doing things in the school and sees their initiatives as a threat to the traditional order. So, they constantly oppose and try to block their efforts.
There could also be a dark magic entity that has infiltrated the school. It preys on the negative emotions in the school environment, and Hermione and Favo Prefect are the ones who stand in its way of spreading chaos. It tries to turn students against them and uses dark magic to create obstacles for them.
A third plot idea might be that there is a competition between different prefects in the school, and Hermione and Favo Prefect are partners. They have to use their combined skills in academics, magic, and leadership to win. Along the way, they have to deal with rivalries, but also form a strong bond that could potentially turn into a deeper relationship.
Draco Malfoy could be a main antagonist. Since Harry becoming a prefect is a sign of prestige and power, Malfoy, who is always jealous of Harry, would try to undermine him. He might spread false rumors about Harry misusing his prefect powers or try to get him in trouble with the professors.
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.
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.
It could be an old and powerful vampire who sees Hermione as a threat to his rule. Since Hermione is a very intelligent witch - now vampire - she might be able to figure out a way to undermine his power.
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 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.
Hades would be a likely main antagonist. He rules the Underworld and might see Hermione as a threat or an opportunity. If he thinks she has something valuable, like her knowledge of magic, he could try to keep her trapped in the Underworld. Or he might oppose her if she's trying to change something in his domain that he doesn't want changed.
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.
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.