It could be a jealous wizard who was once in love with Hermione. Discovering she has a child with Fred, this wizard might seek to harm the child out of spite. He could use dark magic to try and kidnap the child or cause trouble for the family.
One idea could be that the child has unique magical abilities that neither Fred nor Hermione expected. Maybe the child can communicate with magical creatures in a way no one else can. Another plot could be the secrecy around the child's existence causing all sorts of misunderstandings among the Weasley family and Hermione's friends. For example, they might suspect Hermione of being involved in some dark magic when they notice her acting strangely to protect the secret.
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 the child is on a quest to bring back a lost magic that was present in the past but will be crucial for the future. Fred and Hermione are initially skeptical but as they see the child's abilities and the evidence of the disappearing magic, they start to believe. Together, they embark on a journey to find ancient spells and artifacts that can restore the magic, all while dealing with the challenges of time travel and the secrets the child holds.
In fanfiction, a Fred and Hermione secret relationship often adds an interesting twist. Writers might explore how their different personalities interact. Fred's humor and mischievous nature could contrast with Hermione's studious and rule - following side. It could start with a chance encounter in the Hogwarts corridors, leading to shared secrets and a growing bond that they keep hidden from their friends for various reasons, like fear of judgment or disrupting the group dynamic.
Fred could use his knowledge of the Weasley family's magic tricks and inventions. He might have some secret gadget or spell that can break through the barriers or spells that are being used to torture Hermione.
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.
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.