One interesting story could be about a dog OC that has a special power and gets involved in the complex world of Kuroshitsuji. For example, a dog OC that can see supernatural entities and helps the main characters in their investigations. Another could be a story where the dog OC is a stray that forms an unexpected bond with Ciel or Sebastian.
First, think about the appearance of the dog. Decide on its breed, size, and any unique physical features like different colored eyes or a special mark. Then, consider its personality. Is it brave, timid, or mischievous? Next, come up with a backstory. Maybe it was once a pet of a noble family or it was born on the streets. You also need to think about how it fits into the Kuroshitsuji world. What role will it play? Will it be an ally to the main characters or have its own independent story arc?
One of my favorite scenes in Ciel x Lizzy fanfiction is when Lizzy stands up for Ciel in front of others. It shows her loyalty and her growth from a somewhat sheltered girl to someone who can be a protector. Another great scene is when Ciel lets his guard down a little around Lizzy, like sharing a quiet moment in the garden. It's a rare moment of vulnerability for him.
Another idea is to create an alternate universe where Ciel and Sebastian's relationship is different. Maybe in this universe, they are friends from the start instead of having a master - servant contract. This could lead to all sorts of different adventures and interactions between them.
Well, in Kuroshitsuji fanfiction, Ciel and Sebastian's relationship is a major draw. Ciel is a young boy with a dark past and a strong will for vengeance. Sebastian, being a demon, offers his services to Ciel in exchange for his soul. This sets up a very interesting dynamic. Some fanfictions explore how Ciel's vulnerability and need for protection play into his relationship with Sebastian. Others might focus on Sebastian's demonic nature and how he manages to both serve and manipulate Ciel at times. Their relationship can be full of tension, especially when Ciel's human emotions clash with Sebastian's more demonic view of the world.
One popular trope is the 'Forbidden Love' aspect. Since Sebastian is a demon and Ciel is a human, their relationship can be seen as something that goes against the natural order. Another is the 'Power Struggle' trope. Ciel may try to assert his authority over Sebastian, but Sebastian's demonic nature gives him an edge in certain situations. 'Soul Bond' is also common. Given their contract and the connection it implies, fanfictions often explore the deeper spiritual and emotional bond that might develop between them.
Some 'dark kuroshitsuji fanfiction' stories focus on alternative timelines where things take a much darker turn. Maybe Ciel fails in his revenge and descends into complete despair, or Sebastian betrays Ciel in a more sinister way than in the original story. These types of fanfictions often play with the psychological and emotional darkness of the characters, showing how they could be pushed to their limits and beyond.
In many Kuroshitsuji fanfictions, the relationship between Alois and Claude is complex. Claude is Alois' butler, but there's often a power dynamic and a sense of mystery. Sometimes, it can be a relationship of dependency, with Alois relying on Claude for various things like protection and fulfilling his desires. However, Claude also has his own motives which may not always align with Alois' best interests.
In Kuroshitsuji fanfiction, Ciel and Lizzy often have a complex relationship. Lizzy is initially presented as a somewhat naive girl, but she has a deep affection for Ciel. Their relationship can be seen as a blend of childhood friendship and something more. Ciel, on the other hand, is burdened with his revenge and his role as the Earl. Sometimes, in fanfiction, their relationship is explored in different ways, such as Lizzy becoming more aware of Ciel's true self and trying to support him despite his dark secrets.
Start with creating your characters. Even if you're using the existing characters from Kuroshitsuji, you might want to add some new traits or backstories to them. For example, if you're writing about Grell Sutcliff, you could give him a different motivation for his actions. After that, plan out your plot. Think about the beginning, middle, and end of your story. Make sure there are some interesting twists and turns to keep the readers engaged.