Here's a plot idea. The OC's pregnancy is linked to an ancient curse in the Black Butler world. The only way to break the curse is for her to give birth successfully, but there are many obstacles in her way. For example, she has to face off against powerful witches who don't want the curse to be broken. Also, the father of the child could be a mysterious figure from the past who has a connection to Ciel's family. This would add an element of mystery and intrigue to the fanfiction. And the OC's journey through the pregnancy could also be a journey of self - discovery as she uncovers secrets about herself and the world she lives in.
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.
A unique plot could be that the OC is pregnant with a half - demon child. This would create a lot of conflict as demons in Black Butler have their own rules and hierarchies. The OC might be hunted by other demons who see the child as a threat or an abomination. Or perhaps the pregnancy is a prophecy come true, and there are groups trying to either prevent or ensure the birth for their own purposes. The OC could also be a noble in the Black Butler universe, and her pregnancy out of wedlock could lead to a major scandal.
Another plot could involve an OC who is a powerful magician from a foreign land. They come to England seeking a powerful artifact that Ciel also has his eyes on. This would lead to a series of battles and negotiations between the OC, Ciel, and Sebastian, with the OC having their own set of goals and motives that might not be entirely clear at first.
One plot idea could be that the male OC is a detective hired to investigate strange disappearances in the Black Butler world. He starts to suspect that there's a supernatural force at work and teams up with the main characters to solve the mystery. Another idea is that he's a nobleman's son who has a secret identity as a vigilante, fighting against the corrupt nobles. He might cross paths with Ciel and Sebastian when they're on a mission related to the same nobles.
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 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.