One idea could be a story where they go on an adventure together to solve a mystery in their town. They might face various challenges that test their friendship and loyalty.
A really interesting plot could be a time - travel one. Gay Sabastion and Ciel accidentally get sent back in time and have to find a way to get back to their own time. Along the way, they interact with historical figures and events, and their relationship evolves as they rely on each other in this strange new situation.
Perhaps a plot where Gay Sabastion has to protect Ciel from a dark force that is after him. In the process, they discover new things about each other and their own strengths. It could involve some magical elements as well, like Gay Sabastion having some special powers that he uses to fend off the danger.
For Gay Sabastion, you could explore his backstory more. Maybe he had a difficult childhood that made him the person he is today. This could add depth to his character and his relationship with Ciel.
One idea could be exploring how Ciel and Elizabeth handle the news. Maybe Ciel becomes overly protective, trying to keep Elizabeth safe from all possible threats. He could hire extra guards and limit her outings. Elizabeth, on the other hand, might feel a bit stifled but also understand his concern. Another plot could involve how their families react. Elizabeth's family might be overjoyed and start planning a grand celebration, while Ciel's family business and the underworld he's involved in could pose some challenges to the pregnancy, like potential enemies using it as a weakness.
One idea could be that Claude discovers a hidden power within Ciel that even Ciel himself is unaware of. This power could be related to Ciel's past or family heritage. Another plot could be that Claude has to protect Ciel from a new and powerful enemy that is specifically targeting Ciel for some unknown reason. Maybe this enemy has a connection to Claude's past as well.
One plot idea could be the reader being a new servant in the Phantomhive manor. Ciel is initially suspicious but as time passes, he starts to trust and rely on the reader in a unique way. Maybe the reader has a special skill that helps Ciel in his investigations or in dealing with the other nobles.
One idea could be Sebastian protecting Ciel from other alphas who see Ciel as a desirable omega. Another could be Ciel struggling with his omega instincts while trying to maintain his role as the head of the Phantomhive family. Maybe Sebastian and Ciel could go on a journey together where they have to rely on each other and their bond deepens as a result.
One idea could be that fem Ciel has to attend a grand ball and Sebastian helps her get ready in a very intimate way, like choosing the perfect dress and doing her hair, while they share some flirty banter. Another could be that fem Ciel gets into trouble with a rival noble and Sebastian uses his demonic powers in a very creative way to protect her, all while their relationship deepens. And perhaps a plot where fem Ciel discovers a secret about Sebastian's past that makes her see him in a new light and changes the dynamic of their relationship.
For an interesting plot, imagine that Alois and Ciel find themselves in a parallel world where their roles are reversed. Alois is now the more proper and reserved one while Ciel is the more flamboyant and unpredictable. This role - reversal forces them to see things from the other's perspective. They have to adapt to this new world and each other. Maybe in this world, there is a prophecy that only they can fulfill together. As they strive to do so, their relationship evolves. They might start off hating the change but gradually come to accept and rely on each other more than ever.
Another plot could involve the reader being a magical being, like a fairy or a witch, who is on a mission to protect Ciel from a dark magic that is threatening him. This would create a lot of opportunities for magical elements and a relationship that develops as the reader uses their powers to keep Ciel safe. They might have to go on quests together to find magical artifacts to counter the dark magic. It's a plot full of adventure and mystery.
A fanfiction could explore what would happen if Ciel was sent to Hogwarts. He could be sorted into Slytherin due to his cunning nature. He might find the magic there strange yet useful in his quest for revenge. Harry could befriend him and try to show him a different path than revenge, while Ciel could teach Harry some of his skills in investigation and strategy.
One idea could be Female Ciel trying to navigate the underworld of the Black Butler universe while facing discrimination for being a female in a male - dominated world. She might have to prove herself even more than her male counterpart did.