In some Harry Potter fanfiction, ghouls are often portrayed as creepy and somewhat menacing creatures. They might be lurking in the attics of old wizarding houses, like the one at the Weasley's. They are usually depicted as less intelligent than wizards but with an instinct for self - preservation and a certain wildness about them.
One common theme is mystery. Ghouls are often depicted as mysterious creatures in the Harry Potter series, and fanfictions tend to explore that further. For example, their origin and their purpose in the Hogwarts castle are often questioned and explored.
Maybe Harry Potter, as a ghoul, has to hide his true identity from his friends at Hogwarts. He might struggle with his newfound nature while still trying to fight against the dark forces in the wizarding world.
One idea could be a crossover where a character from Harry Potter accidentally enters the world of Tokyo Ghoul. Medusa could be incorporated as a dark force or an ancient evil that has somehow influenced both worlds. Maybe she has the power to turn ghouls into stone, which throws the already complex ecosystem of Tokyo Ghoul into chaos. And in the Harry Potter side, her presence could be related to dark magic spells that are long forgotten.
One idea could be a crossover where a vampire from the Harry Potter universe somehow ends up in the world of Tokyo Ghoul. Maybe a vampire who was hiding in the Forbidden Forest gets transported to Tokyo and has to adapt to the new rules of the Tokyo Ghoul world, dealing with ghouls and the CCG. This could lead to some interesting power dynamics as the vampire's magic might be a new factor in the Tokyo Ghoul ecosystem.
Perhaps the story could start with her transformation into a ghoul due to some dark magic gone wrong. As a female Harry Potter - turned - ghoul, she might then be on a quest to reverse the transformation. Along the way, she could meet other ghouls and learn about their culture and society, all while still having her ties to the wizarding world and the people she cares about there.
Perhaps the plot could involve Fem Harry Potter's transformation into a ghoul being a result of some dark magic experiment gone wrong. She then has to find a way to reverse it while facing discrimination from some in the wizarding community who fear ghouls. Along the way, she could discover new magical abilities related to her ghoul form and use them to her advantage.
It depends on how you define 'popular'. There are so many different types of fanfiction in the Harry Potter universe. Some concepts gain a lot of traction and are shared widely among fans, but others remain more niche. This 'harry potter is a ghoul fanfiction' might be one of those that only a small subset of fans who are particularly interested in unique or darker interpretations are aware of.
The magic system could be used to both combat and understand the ghouls. Spells like Legilimency could be used to read the minds of ghouls to find out their intentions. And defensive spells like Protego could be crucial when facing a ghoul's attack.
One interesting fanfic could be 'The Wizarding World and the Ghouls'. It combines the magical elements of Harry Potter with the dark and gritty world of Tokyo Ghoul. It might have Harry Potter characters somehow transported to the Tokyo Ghoul universe and they have to deal with the ghouls while still having their magical abilities. However, the story might also explore how the two different power systems interact and conflict with each other.