In such a fanfic, Bellatrix might use dark magic or psychological torture to drive Harry insane. She could target his memories of his parents or friends, causing him to lose his grip on reality.
It's not appropriate. The 'Harry Potter' series has a certain moral and ethical framework. Fanfics that involve sexual scenarios between its characters distort this framework. Moreover, Bellatrix is a villain and such a relationship in a fanfic is not only inappropriate but also a strange and unethical interpretation of the characters.
Well, power dynamics is a big theme. Bellatrix is a powerful witch, and Harry has his own unique powers. Fanfics might show how they use or struggle with their powers in relation to each other. Also, the theme of forbidden love sometimes pops up. The idea that Harry and Bellatrix, who are on opposite sides, could have some sort of romantic or at least a very strong emotional connection that goes against the norms of their world.
Well, an insane Harry Potter fanfic might involve Harry being raised by the Malfoys instead of the Dursleys. This would completely change his personality and the way he views the wizarding world. Or it could be a fanfic where the magic in Hogwarts goes haywire and spells start to have opposite effects. For example, a levitation spell makes things sink instead. Also, a fanfic where Harry finds out he has a long - lost twin sister who has been hidden away by some dark magic.
Writing an insane Harry Potter fanfic starts with coming up with a crazy premise. For instance, what if Hogwarts was built on top of an ancient magical portal that leads to other dimensions? Then, do a lot of world - building around this new idea. How does this portal affect the magic in Hogwarts? How do the characters react when they discover it? You also need to give your characters some unique traits or abilities that are different from what we know in the original story. And keep the dialogue interesting and full of surprises, like having characters speak in riddles all the time.
One example could be a fanfic where a character is driven to criminal insanity by a dark magic spell gone wrong. For instance, if a minor character is accidentally exposed to a powerful Imperio-like curse that makes them commit heinous acts against Hogwarts students. Another could be a story where the Dementors' influence on a wizard causes them to gradually lose their sanity and engage in criminal behavior, like stealing powerful magical artifacts from the Ministry of Magic. There might also be a fanfic where a potion experiment turns a character into a criminally insane version of themselves, with their moral compass completely shattered and they start causing chaos in the wizarding world.
In a Bellatrix and child Harry Potter fanfic, the author might explore the idea of Bellatrix having a change of heart towards Harry. It could start with her seeing some of James Potter in the young Harry. This could lead to a series of events where she tries to protect Harry from other Death Eaters, which would be a huge deviation from her character in the original series. However, it would make for a very interesting and unique fanfic.
One great Bellatrix fanfic is 'The Redemption of Bellatrix'. It shows a different side of her, exploring the possibility of her having a change of heart. It's really interesting to see how the author weaves a new story around her character.
In a time - travel Bellatrix fanfic, she might go back to the time when she was first introduced to the Dark Arts. This could show a different side of her, perhaps a more innocent - looking but still dark - inclined Bellatrix. She could interact with younger versions of other Death Eaters and form different alliances that change the course of the future in the wizarding world.