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.
Another plot could involve the school being divided into different factions based on family backgrounds or skills. Ciel has to navigate through this complex social structure with Sebastian by his side. Elizabeth could be the popular girl who has a crush on Ciel and is always trying to get his attention. There could be competitions between the factions, and Ciel's goal is to unify the school in his own way.
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.
A cool plot for 'avengers high school au fanfiction' might involve a mystery. Maybe there are strange happenings in the school like objects floating around mysteriously. Wanda Maximoff could be the one who initially notices and tries to figure it out with the help of Vision. Meanwhile, Black Widow could be the popular girl who is secretly a spy, investigating a possible threat to the school from the inside.
One idea could be Merlin as the new student with mysterious powers. He tries to hide his magic from everyone, but it keeps getting him into trouble, like accidentally making things levitate in class. Another could be a high school version of the Arthur - Merlin relationship, where Arthur is the popular jock and Merlin is the nerdy outcast, but they become friends over time.
One idea could be that the houses are like different cliques in high school. For example, Gryffindor could be the brave and popular kids who stand up to bullies. Slytherin might be the ambitious group that is always vying for the top positions in school clubs. Another plot could involve a high - school version of the Triwizard Tournament, but with normal high - school sports and academic challenges instead of magical ones. And a third idea is that there's a mystery about a hidden room in the school that only certain students can find, and it has secrets related to the founders of Hogwarts.
Alois could be a main antagonist. In a high school setting, he might be the jealous rival of Ciel. He could try to undermine Ciel's position as student council president or spread rumors about him. His butler, Claude, would be helping him with his devious 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.