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 great plot idea for Black Butler Sebastian and Ciel fanfiction could be a story where Ciel gets cursed and starts to forget his memories bit by bit. Sebastian has to race against time to find the source of the curse and break it before Ciel forgets who he is completely, including their contract. Another could be that Sebastian and Ciel find themselves in a parallel universe where the roles of demons and humans are very different. Here, they must find a way to get back to their own universe while also dealing with the new power dynamics and moralities of this strange new world. And, a simple yet interesting plot could be that they attend a grand ball in the demon world, where Sebastian's past is revealed in bits and pieces through the interactions they have with other demons.
Well, one plot idea is that a new powerful demon emerges and challenges Sebastian for Ciel's soul. This forces Sebastian to prove his worth as Ciel's butler and protector. They would have to engage in a series of battles and tests set by the new demon. Another plot could be that Ciel discovers a hidden power within himself that has the potential to free him from his contract with Sebastian. But this power also comes with its own risks and consequences. They have to decide whether to explore this power or not. Also, a plot where they go on a journey to find a rare magical item that can grant Ciel one final wish. This journey would be filled with various obstacles and they would meet different characters along the way.
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 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 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.
One idea could be Ciel exploring his new demonic powers. He might start to see the world in a completely different way, with his values shifting as he is no longer bound by human limitations. For example, he could use his powers to seek revenge in a more extreme way than he could as a human.
One interesting story could be about Ciel and Sebastian going on a journey to a mysterious land where they encounter new challenges that test their relationship. In this fanfiction, they might have to face powerful enemies together and rely on each other more than ever.
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 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.
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.
Perhaps a story where Elizabeth and Ciel time - travel back to a period before all the tragedy in Ciel's life occurred. Elizabeth could then try to prevent the events that led to Ciel's current state. This would involve her getting to know a more innocent Ciel and also dealing with the consequences of changing the past, such as how it might affect their relationship in the present.