The theme of fear is also present. Ciel is afraid of dying before he can achieve his revenge, and this fear is magnified when he's sick. Fanfics might also explore how his sickness impacts his relationship with Elizabeth. She might be more protective or even a bit scared of losing him, which could change the dynamic of their relationship.
One common theme is the vulnerability of Ciel. When he's sick, his usually strong and stoic facade crumbles, and we see a more human side of him. Another theme is the loyalty of his servants. They go to great lengths to care for him, highlighting the bond they have. For example, Sebastian might use his demonic powers in a different way to heal Ciel rather than just for his usual tasks.
One possible fanfic could be about Ciel getting a mysterious illness that makes his servants scramble to find a cure. They might call in various doctors or use their own unique skills to try and heal him. Sebastian, in particular, would be torn between his loyalty to Ciel and his own demonic nature, as he sees Ciel in a vulnerable state.
One common plot is other characters in the 'Black Butler' universe taking care of a sick Ciel. Sebastian, being his butler, often plays a major role in these stories. He might go to great lengths to find a cure or just to make Ciel comfortable while he's ill.
One popular theme is the reader helping Ciel with his revenge plot. Since Ciel is on a mission for vengeance in the original series, many fanfics explore how the reader can be involved in that journey, either by providing emotional support or practical assistance.
One great fanfiction could be where Sebastian tries every possible method to cure Ciel of the flu quickly, but Ciel's stubbornness makes it difficult. Maybe he refuses to stay in bed and rest, causing more trouble for Sebastian as he tries to take care of his young master while also dealing with the manor's affairs.
One common theme is Ciel's revenge. Since his backstory in the original series is about seeking revenge, many fanfictions explore this further. Another is his relationship with Sebastian, so there are a lot of fanfictions that focus on their master - servant dynamic in different situations.
One common theme is the exploration of Ciel's relationship with Sebastian as a demon. How their contract changes and how Ciel adapts to his new demonic nature. Another theme is Ciel's struggle for power in the underworld, as he is now a demon. It often involves him facing other powerful demons.
Another theme is the struggle between Ciel's human side and his demonic nature. Fanfics may show him grappling with his emotions, memories, and moral compass in the face of his transformation. For example, he might question his past actions and how they fit into his new identity. It makes for a very interesting character study.
In a fanfic named 'Farewell, Young Master', Ciel could die in a tragic accident. Maybe he's on a mission and things go horribly wrong. His death would shock the Phantomhive household. Sebastian, being a demon, might have some interesting internal conflicts. He was contracted to Ciel, and now that Ciel is dead, he has to decide what to do next. Does he continue to be involved with the household? Or does he simply disappear? This could be an interesting exploration in the fanfic.
One good 'Black Butler Ciel Pacifier' fanfic could be those that explore Ciel's childhood in a unique way. For example, a fanfic might show how the pacifier could be a symbol of the innocence he lost at a young age. It could also be used in a story where Ciel is transported back to a time when he still used a pacifier and has to deal with the memories and emotions associated with that period.
One common theme is identity. Since Ciel already has a complex identity, adding a twin doubles that complexity. Another theme is family, exploring the relationship between the twins and how it affects their actions.