In this fanfiction, it could be that Fenrir Greyback is actually a double agent. He pretends to be on Voldemort's side but is secretly watching over Hermione to keep her safe from the Death Eaters. This could lead to a lot of tense situations where Hermione is in danger and Fenrir has to use his werewolf skills and knowledge of the Death Eaters to protect her without blowing his cover.
One possibility is that it's a story of redemption for Fenrir. He starts to watch over Hermione and in the process, he begins to see the good in people he's been trying to harm. As he watches her, he might be influenced by her kindness or her intelligence, and this gradually changes his view of the wizarding world and his place in it.
It could be other Death Eaters. They might suspect that Fenrir is up to something with Hermione and try to stop him. For example, Bellatrix Lestrange could be a major antagonist. She's known for her cruelty and loyalty to Voldemort, and she might not trust Fenrir's actions regarding Hermione.
Well, perhaps in the fanfiction, Fenrir Greyback is under the command of Voldemort to watch Hermione as she is seen as a threat to the Dark Lord's plans. He might follow her around Hogwarts, observing her every move. This could lead to some tense moments where Hermione senses she's being watched but can't quite figure out who it is at first. As the story progresses, she could start to set traps to catch her stalker, which would be Greyback. And there could be a big showdown between them in the end.
One possible plot could be that Fenrir, for some reason, has a change of heart. Maybe he sees Hermione's kindness and intelligence during one of his encounters with the wizarding world. He starts to protect her secretly from other dark forces. And Hermione, initially afraid, gradually realizes there's more to him than just a monster.
One possible plot could be that Hermione stumbles upon Fenrir Greyback in a situation where he's about to be killed by some dark wizards who see him as a loose end. Hermione, being the empathetic and brave witch she is, decides to save him. Maybe she believes that every life is worth saving, even a werewolf like Fenrir. She could then try to rehabilitate him, using her vast knowledge of magic and her strong moral compass.
One possible plot could be that Fenrir Greyback, for some reason against his usual nature, discovers Hermione in a vulnerable state. Maybe she's been separated from her friends during a battle. He decides to keep her safe from other dark creatures, seeing some sort of potential in her that he doesn't want wasted. He might teach her some of the ways to survive in the wild, which are skills he has as a werewolf.
One possible plot could be Hermione's research on werewolves leading her to interact with Fenrir Greyback. Maybe she wants to understand the werewolf curse better and tries to interview him in Azkaban. Through their conversations, she discovers the dark and twisted past that led him to become such a feared and cruel werewolf.
A possible plot is that Hermione, in her pursuit of equal rights for magical creatures, decides to reach out to Fenrir Greyback. At first, he is hostile, but Hermione's determination and her ability to see beyond his monstrous side start to change him. They could end up in a situation where they have to work together to stop a greater evil that is targeting werewolves and other marginalized magical beings. This would involve Hermione using her logical thinking and Fenrir using his knowledge of the werewolf underworld.
Their relationship could start with Hermione being a captive of Fenrir. But instead of harming her immediately, he becomes intrigued by her intelligence. As time passes, Hermione might start to see some humanity in Fenrir that others don't, and they could form an unlikely alliance against a common enemy.
Perhaps it's the allure of the 'forbidden'. Fenrir Greyback is a dark and dangerous character, and exploring his relationship with Hermione in fanfiction might be an exploration of the contrast between light (Hermione) and dark (Greyback).
In one fanfiction concept, Greyback saves Hermione when she is trapped in a magical maze created by dark wizards. Greyback, being a cunning and experienced dark creature, is able to navigate the maze and get to Hermione before the maze collapses or she is killed by the various traps inside. He might save her because he senses that she has a great potential that could be useful to him in the future.