You could also write a fanfiction focused on the supernatural elements in Kuroshitsuji. Dive deeper into how the demons, reapers, and other supernatural beings interact with the human world. What if there were more types of supernatural creatures that we haven't seen yet? This could open up a whole new world of possibilities for the story.
Another idea is to create an alternate universe where Ciel and Sebastian's relationship is different. Maybe in this universe, they are friends from the start instead of having a master - servant contract. This could lead to all sorts of different adventures and interactions between them.
A plot could involve Ciel Phantomhive being magically transported to Hogwarts. Since he's used to a world of mystery and intrigue in his own story, he might quickly adapt to the secrets of Hogwarts. He could form an unlikely alliance with Harry Potter and his friends, and together they face a new threat that combines elements from both universes, like a dark wizard who has made a deal with a demon.
One interesting story could be about Sebastian from 'Kuroshitsuji' finding himself in the wizarding world of 'Harry Potter'. He might interact with the Hogwarts students and teachers in unique ways. Maybe he uses his demon powers to solve some of the magical mysteries in the castle.
One interesting story could be about Pluto's hidden past in the Kuroshitsuji universe. Maybe it explores how Pluto came to be in the service of the characters in the series. Fanfiction often takes the known elements of the original work and twists them in unique ways.
One interesting story could be about a dog OC that has a special power and gets involved in the complex world of Kuroshitsuji. For example, a dog OC that can see supernatural entities and helps the main characters in their investigations. Another could be a story where the dog OC is a stray that forms an unexpected bond with Ciel or Sebastian.
Perhaps a fanfiction where Alois and Ciel are transported to a modern - day world. They have to adapt to the new environment and in the process, they find unexpected common ground. Alois is fascinated by the technology, while Ciel is more interested in the new business opportunities. Their relationship evolves as they face challenges together in this strange new world.
Well, perhaps a concept is the power dynamics. If Harry is death in this crossover, he would likely have a great deal of power. In Kuroshitsuji, power is a major theme, especially with Sebastian's demonic powers. Harry's power as death could either clash or complement Sebastian's. This could lead to some really intense and unique storylines where the characters are forced to navigate around these two powerful forces.
Well, some 'Kuroshitsuji Female Ciel' fanfictions might explore her relationship with Sebastian differently. For example, since she's female, there could be a more complex emotional dynamic. In one fanfic, she might be more conflicted about her feelings towards Sebastian, not just as a master - servant relationship but with a touch of romantic tension. Also, there could be fanfictions that focus on her growing up as a female in a male - dominated world of the Kuroshitsuji story, facing different challenges compared to her male counterpart.
Some 'dark kuroshitsuji fanfiction' stories focus on alternative timelines where things take a much darker turn. Maybe Ciel fails in his revenge and descends into complete despair, or Sebastian betrays Ciel in a more sinister way than in the original story. These types of fanfictions often play with the psychological and emotional darkness of the characters, showing how they could be pushed to their limits and beyond.
In terms of plot, these fanfictions might explore different scenarios. For example, a male oc could be inserted into the main storyline, perhaps becoming an ally or a rival to the main characters. This can lead to interesting twists and turns as they interact with Sebastian and Ciel.
Some fanfictions explore the concept of Sebastian's true nature as a demon and how it affects Ciel. For example, his demonic abilities might both protect and endanger Ciel at the same time. The theme of revenge also comes up a lot. Ciel's quest for revenge and how Sebastian aids him in it can be a central plot. There's also the theme of sacrifice. Whether it's Ciel sacrificing something to keep Sebastian by his side or vice versa, it adds an interesting layer to the stories.