A really cool plot could be that there is a high - school tournament, and Ciel enters it to prove his worth. But this tournament has some dark undertones as it was actually set up by a group of demons who want to test Ciel's strength. Sebastian, of course, is there to make sure Ciel doesn't get into too much trouble and to help him win the tournament in his own devious ways.
Perhaps in the fanfiction, the high school is divided into two factions like in the original with the demons and those who oppose them. Ciel has to navigate through high school life while also dealing with this power struggle. He could make friends who are actually on different sides of this hidden battle, and Sebastian helps him out in various ways, like getting him out of tricky situations with teachers or rival students.
One idea could be that Ciel is the new student council president, trying to reform the school's old - fashioned rules while dealing with a rival group led by Alois. Sebastian could be his loyal butler and also a star student in various subjects, secretly helping Ciel with his plans.
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.
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 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.
A great plot could be that the triplets are given the task of protecting a special guest in the Black Butler universe. They have to deal with various threats while also trying to figure out the guest's true motives. They use their combined intelligence and physical abilities to keep the guest safe.
One interesting plot could be the main characters adapting to high school rules while still maintaining their unique 'Black Butler' characteristics. For example, Sebastian might use his skills to excel in home economics class, making perfect tea and pastries. Another plot could involve the students uncovering a mystery within the high school that has ties to the supernatural world, just like in the original 'Black Butler' series.