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.
Well, perhaps Hermione would meet her old Muggle school friends and be a bit of an enigma to them. They might notice how much she's changed but not be able to put their finger on it. She could have some interesting conversations about her new life at Hogwarts and her adventures, while still keeping the magical world a bit of a secret. There could also be some comical moments when she tries to fit back into the Muggle world and forgets some of the Muggle ways she used to know.
In the fanfiction, Hermione's Muggle friends could be brought into the wizarding world during the reunion. They could meet some of Hermione's wizard friends like Harry and Ron. There could be a lot of cultural clashes and interesting conversations as the Muggle friends learn about wizards and vice versa. For example, they might be shocked to learn about Quidditch and how it's played in the air.
One idea could be that Hermione invites her Muggle friends to a magical place for the reunion. They could be amazed by the wonders of the wizarding world, like seeing Hogwarts for the first time. Hermione would have to explain all the magical things around them, and there could be some funny misunderstandings as her Muggle friends try to adjust to this new and strange environment.
She might meet her old teachers. They would be surprised to see how much she's grown and changed. And of course, her former classmates. There could be the popular girl from school who is now a bit jealous of Hermione's confidence. Or the nerdy boy who has now become a successful scientist and they can have an interesting exchange about different kinds of knowledge, magical and Muggle.
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.
Perhaps a former teacher who always had it in for Pete. At the reunion, this teacher could be there as a guest and start criticizing Pete's life choices or bring up old mistakes he made in school. The teacher might be jealous of Pete's new - found confidence or success and wants to put him down.
It could be a new Decepticon who is trying to stop Charlie and Bumblebee from reuniting. This Decepticon might see their reunion as a threat to their evil plans.
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.
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.