One idea could be a story where Claude and Hannah are forced to work together on a mission by some powerful entity in the Black Butler universe. They start off distrusting each other but gradually develop an understanding. For example, they might have to retrieve a powerful artifact that could change the balance of power between demons. Another plot could be a pre - Black Butler story, showing how they first met and the early days of their relationship, which was full of tension and unspoken attraction.
Well, a plot could revolve around Hannah's loyalty to her master and how Claude tries to use that loyalty to his advantage at first. But as time goes on, he finds himself truly caring for her in a way he didn't expect. Maybe they are in a situation where they have to protect a human from other demons together. This would put them in a position where they have to rely on each other's skills and powers. And as they face various challenges, their relationship deepens.
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.
Well, Claude's personality is marked by his efficiency and his somewhat aloof nature. He is used to getting what he wants through his own means. Hannah is a more emotional and dedicated character. In a fanfic, their different personalities can be explored through different scenarios. For instance, in a situation where they have to make a decision about helping a human or not. Claude would weigh the pros and cons from a self - interested perspective, while Hannah would consider it from a moral or loyalty - based angle. This difference can lead to arguments and debates between them, but also opportunities for them to learn from each other and grow closer as they understand each other's viewpoints better.
There's also 'The Enigmatic Bond of Claude and Alois'. This fanfic focuses on the more mysterious aspects of their connection. It's full of detailed descriptions of their interactions and the power dynamics between them, which is really fascinating for fans of the pairing.
One idea could be a story where a new character with a black and red aesthetic enters the 'Black Butler' world. This character could be a rival to Sebastian, with their own dark secrets and powers. For example, they might have the ability to control the red flames of hell, just as Sebastian has his demonic powers.
One interesting plot could be Undertaker's past being explored in more detail. Maybe he had a relationship with one of the other main characters' ancestors that comes to light and affects the present story. For example, if he had a connection with Ciel's family long ago.
One idea could be that Ciel has finally achieved his revenge, but now he finds himself lost without a purpose. He then discovers a new threat to the underworld that he feels compelled to fight against. This could involve new characters and a whole new set of mysteries.
One idea could be an OC who is a detective from another country coming to London to solve a series of mysterious murders that seem to be linked to the supernatural activities in the Black Butler world. This OC could cross paths with Sebastian and Ciel as they are also investigating the same case from their own motives.
In 'Black Butler Sebastian x Claude fanfic', one of the main themes is the exploration of their complex personalities. They are both mysterious and have their own motives. This is often explored in detail. Another theme is the idea of loyalty. Their loyalty to their masters can create interesting conflicts and situations. And a third common theme is the concept of the supernatural world they inhabit. The rules and hierarchies of the demon world are often part of the stories.
Another theme is obsession. Alois is often depicted as being obsessed with Claude in some way, whether it's for protection or something more sinister. And Claude has his own reasons for being fixated on Alois. It's a very dark and twisted form of mutual obsession in many of these fanfics.
One interesting plot could be a competition between Claude and Sebastian that goes beyond their duties to their masters. For example, they might compete in a series of challenges set by an otherworldly entity. Another plot could involve them having to team up against a common enemy that threatens the very existence of their world. Maybe a powerful demon from the depths of hell who wants to enslave all reapers and demons.