What are some interesting plot ideas for Black Butler OC fanfiction?

One idea could be an OC who is a detective from another country coming to London to solve a series of mysterious murders that seem to be linked to the supernatural activities in the Black Butler world. This OC could cross paths with Sebastian and Ciel as they are also investigating the same case from their own motives.

One idea could be to create a demon OC who was once a human with a tragic past, like a wronged artist. The OC could have unique demonic powers related to art, such as being able to bring paintings to life in a twisted way. Another idea is a demon OC that is an outcast among demons because they have a sense of morality that is different from the norm in the demon world. They could befriend Ciel in an unexpected way and help him not just for the soul contract but out of genuine care.

One plot idea could be that the OC is a new servant in the Phantomhive household. Sebastian initially views her as a nuisance but as they interact more, a strange attraction develops. For example, during a ball at the manor, the OC accidentally spills wine on a noble guest. Sebastian steps in to save the situation and in the process, he realizes her charm and quick - wittedness.

One idea could be that the OC's pregnancy is a result of some dark magic experiment gone wrong in the Black Butler world. And she has to find a way to reverse it while protecting the unborn child. Another is that the baby has some special powers that different factions in the series are after. Maybe the OC is a servant in the Phantomhive household and the pregnancy causes a stir among the other servants.

One idea could be a new character coming into the Phantomhive household as a potential mate for Sebastian. This character could be a half - demon with unique powers and a mysterious past. Their presence would disrupt the usual dynamic between Sebastian and Ciel, creating all sorts of interesting conflicts and power struggles.

One idea could be creating a story where a new character arrives at the Phantomhive manor with a special power that Ciel is initially suspicious of but later realizes can be useful in his battles against his enemies. For example, the power to communicate with the dead, which could provide valuable information in their investigations.

One idea could be exploring their past before the events of the main story. Maybe a story about their early training or a secret mission they had to undertake together as kids.

A cool plot for a 'Black Butler' crossover fanfiction might be a crossover with 'Kuroshitsuji' and 'Death Note'. Light Yagami could somehow cross paths with Ciel and Sebastian. Light might see Ciel as a means to an end in his grand scheme of creating a new world order, while Sebastian would be intrigued by Light's Death Note and the power it holds. It would be a battle of wits between them, with Ciel caught in the middle, trying to figure out how to use this situation to his advantage.

Well, you could write a fanfiction where the Black Butler characters are in an alternate universe. For example, a steampunk world. Sebastian could be a mechanic instead of a butler, and Ciel could be a young inventor. Or perhaps a story where Grell becomes Ciel's butler for a while, which would be really chaotic and full of humor. And then there's the idea of a story where the Phantomhive Manor is haunted by a different kind of spirit, not just the usual demons and ghosts that are part of the original story.

One idea could be exploring Pluto's past before he became a demon. Maybe he had a family or a purpose that was lost. Another could be a story where Pluto forms an unlikely friendship with one of the other characters in the Black Butler universe, like Mey - Rin. It could be interesting to see how their different natures interact.