Well, perhaps the pregnancy is a symbol of a new beginning in their relationship. In the fanfiction, Sebastian and Ciel might have a journey where they both grow and change because of the impending birth. Sebastian could start to feel real emotions towards Ciel and the baby, which is a big change from his usual demon self. And Ciel, on the other hand, might become more vulnerable yet also more determined to protect what he has now. The story could also explore how the other characters in their world react to this unexpected situation.
One possible storyline could be that Ciel somehow gets magically pregnant in a world where such things are possible in the fanfiction realm. Sebastian, being bound to Ciel, is then faced with protecting Ciel and the unborn child from various threats, like other demons who might see the child as a powerful entity. Another could be that the pregnancy is a result of a curse, and they have to find a way to break it before the birth. It could also involve Ciel's enemies using the pregnancy as a way to manipulate him.
A storyline could be that there is a prophecy about a child born from Ciel and Sebastian that will change the balance of the world. This leads to different factions either trying to protect or destroy Ciel during his pregnancy. Sebastian has to use all his powers and cunning to keep Ciel safe. Also, the fanfiction could explore how Ciel deals with the physical and emotional changes of pregnancy while still being the head of his household. He might have to rely on Sebastian more than he ever wanted to, which could create a whole new dynamic between them.
At first, Sebastian may be confused. Demons don't typically deal with pregnancy. However, his loyalty to Ciel would make him take on the role of a protector very seriously. He would be constantly vigilant, making sure Ciel has proper nutrition and rest. He might also start to feel a strange sense of possessiveness over Ciel and the unborn child, which is a new emotion for him as a demon. And he would have to deal with his own internal struggle as his demon nature clashes with this newfound care for Ciel and the pregnancy.
A pregnancy plot in a Sebastian - Ciel Paris fanfic would be a very unique take. It would likely involve a significant departure from the original story's themes. Maybe the Paris setting could add some elements of mystery and romance. For example, they could encounter a powerful witch in Paris who casts a spell on Ciel that has unforeseen consequences related to pregnancy. Or perhaps in this fanfic, Sebastian and Ciel have a different kind of relationship that develops over time in Paris, leading to a situation where a pregnancy could be a part of the story. But it would definitely be a controversial and unexpected plot development.
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 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.
One plot idea could be that the OC is a new servant in the Phantomhive household. Ciel is initially suspicious of the OC, but as time goes on, the OC proves to be very helpful in solving mysteries around the estate. Sebastian observes the OC's actions and there's a sort of friendly rivalry between the OC and Sebastian in terms of who can serve Ciel better.
Another idea for a fanfiction is that female Ciel is kidnapped by a rival group, and Sebastian has to go on a rescue mission. Along the way, he discovers new aspects of her personality that he hadn't noticed before, and this changes their relationship dynamic as they face various challenges during the rescue.
In this fanfiction, it could be that a powerful force in the underworld wants to see what would happen if a human and a demon had a child. So, it manipulates events to make Ciel pregnant. Ciel and Sebastian would initially be shocked but then gradually accept the situation. The story could detail how Sebastian protects Ciel during the pregnancy, how they prepare for the baby's arrival, and how they raise the child in a world that is not so accepting of their unique family situation. It could also show the growth of their relationship as they face this new chapter in their lives together.
Well, in non - lemon fanfic, Sebastian is often depicted as a sophisticated, powerful demon who is bound by contract to serve Ciel. Ciel is a young, determined noble with a dark past. But in lemon fanfic which is inappropriate, it may distort their characters in improper ways for the sake of adult - themed content. So it's better to focus on their proper character traits from the original source like Sebastian's loyalty (in a non - inappropriate sense) to the contract and Ciel's intelligence and drive to seek revenge.