In a future fanfic, the Queen could be in a more vulnerable position. Ciel and Sebastian are called upon to not only protect her but also to deal with a political conspiracy within the royal court. This would bring in a lot of political intrigue and power plays, with Ciel using his wits and Sebastian his demonic powers to navigate through it all.
Maybe Sebastian and Ciel end up in a modern - day world by some sort of magic mishap. Ciel has to adapt to the new technology while Sebastian tries to figure out how to get them back to their own time. They could get involved in modern - day crimes that are somehow related to the supernatural world they come from.
One idea could be that Ciel has finally achieved his revenge, but now he finds himself lost without a purpose. He then discovers a new threat to the underworld that he feels compelled to fight against. This could involve new characters and a whole new set of mysteries.
One idea could be a story where a new character with a black and red aesthetic enters the 'Black Butler' world. This character could be a rival to Sebastian, with their own dark secrets and powers. For example, they might have the ability to control the red flames of hell, just as Sebastian has his demonic powers.
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 interesting plot could be Undertaker's past being explored in more detail. Maybe he had a relationship with one of the other main characters' ancestors that comes to light and affects the present story. For example, if he had a connection with Ciel's family long ago.
One idea could be that Sebastian's sneeze is a sign of a strange magic affecting him. For example, a spell has been cast on him that makes him have an uncontrollable sneezing fit whenever he gets near a certain magical object. This could lead to comical situations as he tries to hide his sneezes while serving Ciel.
One idea could be a fanfic where the kittens in Black Butler have special powers. For example, they can communicate with spirits and help Sebastian solve mysteries that are related to the supernatural world in the Black Butler universe. Another idea is a story about the kittens' origin, perhaps they were created by some ancient magic long forgotten in the series' lore. They could be key to unlocking a hidden power or secret within the Phantomhive manor.
A cool idea for a fanfic could be that the reincarnation is not just of a human, but of a demon as well. A demon from a different realm is reincarnated into the Black Butler world and has to adapt to the rules and power dynamics here. This could lead to some really unique interactions with Sebastian and the other demons in the series.
You could write a fanfic where the Black Butler characters react to different art forms. Suppose they visit an art museum in a modern setting. Sebastian, with his refined taste, would have in - depth critiques of the paintings and sculptures, while Ciel might be more drawn to the darker or more mysterious artworks. Or they could react to a live theater performance, with Ciel being amazed at the actors' skills and Sebastian observing the behind - the - scenes details like the stagecraft and the actors' emotions.
One idea could be a story where Claude and Hannah are forced to work together on a mission by some powerful entity in the Black Butler universe. They start off distrusting each other but gradually develop an understanding. For example, they might have to retrieve a powerful artifact that could change the balance of power between demons. Another plot could be a pre - Black Butler story, showing how they first met and the early days of their relationship, which was full of tension and unspoken attraction.
One plot idea could be that the reader is a new servant in Alois' household. They have a special ability, like being able to communicate with animals. This causes Alois to be intrigued, and they start to form a friendship as the reader helps Alois understand the secrets of the manor that are related to the animals around it.