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.
There's a fanfiction called 'Greyback's Redemption Arc'. In it, Fenrir Greyback saves Hermione from a group of Death Eaters. The author might have written it to explore the idea of redemption for a character like Greyback. Maybe he saves Hermione because he sees her as a symbol of the good in the wizarding world that he has lost touch with, and this act of saving her could be the start of his journey towards redemption.
Hermione's character could develop in terms of her understanding of the grey areas between good and evil. Saving Fenrir goes against the common perception of him as a monster. This could make her more open - minded and less judgmental. She might start to see that even the most feared creatures can have a chance at redemption.
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.
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.
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).
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.
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.
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.
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.
I'm not sure if there is such a specific fanfiction. It seems like a rather dark and unusual pairing considering Fenrir Greyback is a werewolf known for his brutality. However, the world of fanfiction is vast, and there might be some out there that explore this concept, but it's not a common or mainstream idea in the Harry Potter fanfiction community.