One idea could be that Sebastian sacrifices himself to save Ciel from a powerful dark magic. In the fanfiction, as he faces death, he reflects on his time with Ciel and how it has changed him from a purely demonic being. He might have an internal monologue about the strange new emotions he has experienced.
A plot could involve a curse placed on Sebastian that slowly drains his life force. As he nears death, he starts to lose his demonic powers. This would be a great opportunity to show his struggle, not only with the physical pain but also with the loss of his identity. He may also have to rely on Ciel more than ever, and this could change their relationship dynamic in unexpected ways.
Perhaps Sebastian is poisoned by an enemy who has discovered his weakness. As he succumbs to death, he tries to ensure Ciel's safety one last time. This could lead to a very emotional scene where Ciel realizes how much Sebastian has truly come to care for him. The fanfiction could explore the concept of mortality for a demon, which is quite an interesting angle.
One idea could be Paula discovers Sebastian's true nature as a demon but instead of being afraid, she tries to use it to her advantage in the complex world of the Black Butler. Maybe she asks him to help her gain more influence in the household or among the nobility. Another plot could involve a love triangle where Paula has feelings for Sebastian, but Sebastian is solely focused on his master, and this creates a lot of tension and drama.
One plot idea could be Sebastian falling ill due to a curse placed on him by a rival demon. He would struggle to maintain his usual composed and efficient self while still trying to serve Ciel. Another could be that he gets a mysterious illness that makes him lose his demonic powers temporarily, and he has to rely on human means and Ciel's help to recover. Maybe he gets sick from consuming something in the human world that his demon body is not used to, like a poisoned food item that affects him in strange ways.
One interesting plot idea could be a time - travel scenario. Sebastian and Ciel are sent back in time to a period in the Phantomhive family's history that is shrouded in mystery. They have to navigate the past, deal with their ancestors, and also face the fact that their actions in the past could change the present they came from. Another idea is a role - reversal. Ciel becomes the demon and Sebastian the human servant, which would completely turn their relationship dynamics on its head and lead to all sorts of comical and also thought - provoking situations. A third plot could be a crossover with another popular anime world, like they end up in the world of Naruto and have to adapt to the ninja way while still maintaining their own goals and relationship.
A plot could revolve around Female Ciel being kidnapped by a rival family who wants to use her to gain power. Sebastian then has to use all his skills to rescue her without breaking the terms of their contract. During her captivity, Female Ciel could learn more about the true nature of her enemies and find a way to turn the situation to her advantage. Or perhaps there is a ball where Female Ciel is expected to find a suitor. But she has no intention of getting married as it would interfere with her goals. Sebastian has to help her fend off unwanted suitors while also dealing with the demons and other dark forces that lurk in the shadows of the ballroom.
One plot idea could be Sebastian falling in love with a human who has the power to see demons' true forms. This creates a unique dynamic as he has to protect her from other demons while also dealing with his growing feelings. Another idea is a time - travel plot where Sebastian goes back in time and meets the love interest in a different era, perhaps in a more innocent time for him. And a third could be a story where the love interest is a powerful witch who challenges Sebastian's loyalty to Ciel in the context of their relationship.
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 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.
One plot idea could be that Sebastian's daughter has to hide her true identity in the human world while secretly trying to understand her demonic heritage. Another could be her getting involved in the affairs of the Phantomhive household in a way that both helps and complicates Ciel's plans. Maybe she has a special power that she's trying to control, which sets her on a journey of self - discovery.
One plot idea could be that the OC is a new servant in the Phantomhive household. Sebastian initially views her as a nuisance but as they interact more, a strange attraction develops. For example, during a ball at the manor, the OC accidentally spills wine on a noble guest. Sebastian steps in to save the situation and in the process, he realizes her charm and quick - wittedness.
One interesting 'Black Butler Sebastian' fanfiction could be those that explore his past before becoming a demon. For example, there might be stories about his life as a human and what led him to make the pact that transformed him. Another type could be fanfictions that focus on his relationship with Ciel in alternate universes, like a modern - day setting where they have different dynamics. There are also fanfictions that give Sebastian more human - like emotions and struggles, which can be really engaging to read as it adds a new layer to his character.