No, it's not likely to be a popular theme. Fenrir Greyback is a menacing and often feared character in the Harry Potter world, and Hermione is a well - loved and positive character. Combining them in such a way as 'fills up' (which is rather ambiguous and could be interpreted in inappropriate ways) doesn't seem like something that would appeal to the general fanfiction audience who usually prefer more positive or at least more in - line - with - the - spirit - of - the - original - work stories.
In the context of fanfiction, 'Greyback fills up Hermione' is a rather strange and concerning phrase. If it's not meant in a literal or inappropriate way, it could be some sort of symbolic or metaphorical use within a very niche and perhaps even a non - canon - compliant fanfic. Maybe it's about Greyback's influence or presence overwhelming Hermione in a story, but without more context, it's hard to say exactly.
In some fanfictions, the relationship between Hermione and Greyback might be a dark and twisted one. It could be used to show the contrast between good and evil in the wizarding world. However, this also poses challenges in terms of character development. Writers need to be careful not to stray too far from the established personalities of the characters in the original Harry Potter series.
It's quite controversial. Fenrir Greyback is a villainous character who preys on others, especially children. Hermione represents all that is good in the wizarding world - intelligence, friendship, and justice. Combining them in a'mate' relationship in fanfiction distorts the values of the original series. However, some might explore it to show extreme character arcs or alternate universes, but it's not a common or uncontroversial theme.
I really liked 'Hermione's Encounter with Greyback'. In this fanfiction, Hermione is put in a difficult situation where she has to outwit Greyback. The author does an amazing job of showing Hermione's intelligence and resourcefulness. It's a short but engaging read.
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 common theme is danger. Since Greyback is a werewolf, there's always the threat he poses to Hermione. For example, in many stories, she has to constantly be on guard against his predatory nature.
The theme of the outsider is often present. Hermione, as a Muggle - born in the wizarding world, and Greyback, as a werewolf shunned by society, are both outsiders in a way. Fanfictions may explore how they connect on that level. Additionally, the theme of mystery is common. There's always the mystery of what Greyback will do next and how Hermione will react, which keeps readers on the edge of their seats.
A possible plotline could be Hermione getting captured by Greyback's pack. She then has to use her wits and magic to escape. Maybe she befriends a young werewolf within the pack and uses that connection to her advantage.
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.