One common theme could be redemption. Since Fenrir is a rather dark character, the fanfiction might explore the idea of him being redeemed through his relationship with Hermione. Another theme could be the struggle between good and evil within the relationship. Hermione represents good and Fenrir represents evil, and their relationship could be about how these two forces interact.
A frequent theme is the power dynamic. Fenrir has a certain power as a werewolf and Hermione has her own intellectual power. The fanfictions might play with how these powers interact in a romantic relationship. Also, there could be themes of acceptance. Hermione accepting Fenrir's werewolf nature and Fenrir accepting Hermione's Muggle - born status in a world that often discriminates against it.
Common themes include the clash of two different worlds. Fenrir comes from a world of darkness and savagery as a werewolf, while Hermione is from the civilized and academic world of Hogwarts. Their relationship in fanfiction might explore how they bridge this gap. There could also be themes of forbidden love, as their relationship would be considered highly unusual and likely frowned upon in the wizarding society. Some fanfictions may also focus on Hermione's attempts to understand and tame Fenrir's wild side.
There might be some out there, but it's a rather unusual pairing considering Fenrir Greyback is a werewolf with dark tendencies in the Harry Potter universe and Hermione is a heroic and intelligent witch. However, some fanfiction writers like to explore unique and 'what - if' scenarios. You could start by looking on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
One common theme could be the power struggle. Since Fenrir is a werewolf and Hermione is a powerful witch, their relationship in the fanfiction might revolve around the clash of their different powers and how Hermione tries to resist Fenrir's obsession. Another theme could be the idea of redemption. Some stories might explore whether Fenrir can be redeemed through his relationship with Hermione.
One key theme is danger. Fenrir is a dangerous werewolf, and his following Hermione puts her in constant peril. Another theme is the power of intelligence. Hermione uses her brains to try and figure out his motives and stay safe.
One possible theme is redemption. Maybe the fanfiction explores the idea of Greyback being redeemed through his interaction with Hermione. This could be appealing as it shows the power of goodness (represented by Hermione) over evil.
There could also be a fanfiction where Hermione Granger is forced to face Fenrir Greyback alone in a dangerous situation. This would showcase her bravery and her ability to use her magic skills under extreme pressure. The story could focus on her mental and magical struggle against Greyback, perhaps with her using some of the more advanced spells she has learned over the years to defend herself and those around her from the werewolf's attacks.
One common plotline could be Hermione finding ways to outwit Fenrir Greyback. For example, she might use her intelligence and knowledge of magic to set traps or create diversions. Another could be other characters coming to Hermione's rescue, like Harry and Ron realizing she's in danger and using their skills to protect her from Greyback.
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.
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.
One reason could be the contrast between them. Fenrir is a dark, savage werewolf and Hermione is a smart, strong - willed witch. Writers like to explore the dynamic of such opposite characters being drawn to each other. It creates a lot of potential for conflict and drama in the story.