One common theme is the clash of two very different worlds. Harry Potter's world is full of magic, friendship, and the fight between good and evil in a more traditional sense. Jeff the Killer, on the other hand, comes from a horror world. So, the fanfics often explore how these two worlds interact. Another theme could be the corruption of the magical world by Jeff's dark and violent nature. For example, he might start to spread chaos in Hogwarts.
Another theme is often about the psychological aspect. How the female Jeff's mind works, what drives her to kill. It might also explore her backstory more deeply, like if she had some sort of trauma in her past that led her to become a killer. Fanfictions often go into detail about her inner thoughts and emotions, which makes it different from the original male Jeff the Killer concept.
Another theme is the struggle between Jeff's dark nature and his newfound feelings. His love for someone might conflict with his urge to kill. This creates a lot of internal and external drama in the stories. Like in a lot of fanfictions, Jeff has to fight his own instincts to protect the person he loves.
One common theme is redemption. Given their dark and violent pasts, many fanfictions explore the idea of either Jeff or Jane seeking redemption through their relationship. Another theme is the power of love to change, as they are two very damaged individuals and love might be the thing that can transform them.
One common theme is the struggle between good and evil. Jane is often portrayed as more of the 'good' side trying to stop Jeff who is seen as the 'evil' one. So there's a constant battle of morals.
One common theme is the exploration of the 'outsider' status. Since both are figures from horror, they are outcasts in a sense. Fanfictions often use their relationship to show how two beings that don't fit into the normal world can find connection with each other.
One common theme is the battle between good and evil within their relationship. Since Jeff is a killer and Jane is often portrayed as more of an anti - hero or hero figure, their interactions can be seen as a struggle between these two forces.
One common theme is fear. Since Jeff the killer is a horror character, the fanfictions often play on the reader's fear of him. Another theme is the idea of an unlikely connection. For example, the reader might be drawn to Jeff's dark side in some way.
One common theme is family. It often explores the relationship between Jeff and his sister in the context of his transformation into a killer. Another theme could be protection, like how the sister might try to protect Jeff or vice versa. Also, there's the theme of horror and how she copes with the terrifying world Jeff has become a part of.
One common theme could be the exploration of their dark and twisted natures. Since both characters are from the horror genre, their fanfictions might focus on their evil deeds and how they interact in a world full of chaos.
One common theme is the exploration of the 'forbidden' or 'taboo' relationship. Since Jeff is a killer, a relationship with an OC is not something that would be considered normal or acceptable in mainstream society. Another theme is the idea of redemption. As mentioned before, the OC might be the catalyst for Jeff to change his ways. Fear and danger are also prevalent themes. The OC is constantly in a state of fear due to Jeff's nature, but at the same time, there is an allure because of his mystery and power.