Well, a plot could be centered around fem Ciel having to make a difficult decision regarding the future of her estate. Sebastian, of course, is there to guide her. But in the process, they start to realize their feelings for each other are more than just master - servant. Maybe there's a plot where an old acquaintance of Sebastian shows up and tries to take fem Ciel away from him. Sebastian then has to fight, not just physically but also in terms of wits, to keep her by his side.
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 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.
The relationship can be developed through shared secrets. For example, fem Ciel could confide in Sebastian about her fears for the future of her family's name. Sebastian, in turn, could share some small details about his life in the demon realm. This would create a deeper bond between them. Another way is through physical proximity. Simple touches like Sebastian helping fem Ciel down from a carriage or adjusting her shawl can build sexual tension and a sense of closeness.
One interesting plot could be Female Ciel using her wits and charm to outwit her enemies with Sebastian by her side. For example, she might be involved in a complex political intrigue in Victorian society, and Sebastian helps her navigate through the dangerous web of lies and power struggles. He uses his demonic powers subtly to protect her while she tries to assert her influence.
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.
The theme of mystery is quite prevalent. There are always secrets between Sebastian and Ciel. Sebastian's true nature as a demon and his long - term plans are often a mystery. In fanfiction, this mystery is sometimes deepened or resolved in different ways. And the theme of sacrifice can also be found. Ciel might have to sacrifice parts of his humanity to achieve his goals with Sebastian by his side, and Sebastian may also have to sacrifice some of his demonic desires for the sake of the contract.
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.
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 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 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.