Maybe the OC is a foreign noble who comes to visit Ciel. The OC has some unique skills or knowledge, like being able to speak to animals which is something Ciel and Sebastian find both fascinating and potentially useful. They all team up to deal with a threat to the Phantomhive name that involves some dark magic and strange creatures. Throughout the story, the OC forms a bond with both Ciel and Sebastian, not a romantic one but more of a comrades - in - arms type of 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.
I'm sorry, I don't have specific fan - fictions to recommend directly. But you can try looking on fan - fiction websites like Archive of Our Own. Search for stories that involve Ciel, Sebastian and original characters (OC) with tags like 'platonic' or 'friendship' to find non - adult - related stories.
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.
In non - adult 'ciel and sebastian' fanfics, Ciel is often depicted as a strong - willed and intelligent young master. He is resourceful and determined in achieving his goals. Sebastian, on the other hand, is the ever - loyal and highly capable butler. Their relationship is one of master - servant with an underlying layer of mystery and mutual dependence. Sebastian's supernatural abilities add an element of the extraordinary to their interactions, and Ciel's past traumas shape his present actions and decisions.
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.
Perhaps there could be a school project where they have to work together. Ciel is really good at academics but lacks confidence in social situations. Sebastian, on the other hand, is great at getting people to do what he wants. They start off not getting along but gradually develop feelings as they work on the project. The high school setting can provide a lot of opportunities for drama, like rivalries from other students or strict teachers who try to keep them apart.
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.
There are many great non - forced fanfictions about Sebastian and Ciel. You could start by looking on popular fanfiction platforms like Archive of Our Own. Search for tags such as 'gen' (which means general, not romantic or adult - themed) and 'platonic relationship'. Some works might focus on their adventures in solving mysteries in the Kuroshitsuji universe, highlighting their master - servant dynamic in a more innocent and story - driven way.
One plot idea could be that Sebastian discovers Ciel's praise kink during their daily interactions. Maybe Ciel blushes or shows a different reaction when Sebastian accidentally gives him a compliment. This could then lead to Sebastian using praise as a way to manipulate Ciel or to strengthen their bond in a more complex way.