It could be a rival student from Hogwarts. Maybe someone who is jealous of Hermione and Ginny's friendship or their popularity. This student could try to sabotage their shopping trip by causing trouble in the shops they visit, like making items disappear or spreading false rumors about the things they buy.
There could also be a powerful, corrupt magical creature as the antagonist. This creature might have some grudge against humans in general or specifically against Harry and Ginny because of their past actions. It could use its magic to make them disappear and keep them trapped in its domain.
Another possibility is an ancient evil that has been awakened. This entity might see Hermione as a threat because of her potential power. So it curses her to stop her from growing stronger. In this case, the antagonist would be very powerful and mysterious, and Hermione and her friends would have to do a lot of research and exploration to find a way to defeat it.
Well, they might be shopping for Quidditch gear. Ginny, being a Quidditch enthusiast, could be dragging Hermione along to find the latest broom accessories or team uniforms. Along the way, they get into a bit of trouble when they accidentally stumble upon a shop that sells dark or restricted items. They have to figure out how to get out of there without getting in trouble with the Ministry. And perhaps they end up finding a really cool, legal Quidditch item that helps Ginny's team win their next match.
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 Draco Malfoy. In some fanfictions, Draco might be pushed to such an extreme by his family's beliefs that he resorts to physical violence against Hermione. He might punch her out of jealousy or to assert his so - called superiority.
Voldemort could be the antagonist. Even if he was defeated in the original story, there could be a scenario where some remnant of his dark magic causes Harry and Hermione to disappear. Maybe he had a secret plan that was set to activate after his death and it involved taking them out of the picture.
The antagonist could be a magical creature that has infiltrated Hogwarts. Maybe it's a mischievous imp that targets Hermione because it senses her strong magical aura. The imp then causes the homework sabotage and tries to make it look like Hermione is responsible. Or it could be an older student who feels threatened by Hermione's academic success and decides to set her up by making it appear she's sabotaging homework.
It could be a jealous wizard who wants to steal Fudge's lemon - related magic for his own glory. This wizard might try to frame Hermione for stealing it first so that he can get her out of the way.
One potential main antagonist could be a disgruntled professor. This professor might feel that Hermione is too much of a know - it - all and decides to deage her as a form of punishment. Or it could be a Death Eater who has infiltrated Hogwarts. They know that Hermione is a valuable asset to Dumbledore's side, so they deage her to weaken the opposition. The Death Eater could then use the deaged Hermione as a pawn in their evil plans.