One interesting story could be about Sebastian from 'Kuroshitsuji' finding himself in the wizarding world of 'Harry Potter'. He might interact with the Hogwarts students and teachers in unique ways. Maybe he uses his demon powers to solve some of the magical mysteries in the castle.
A great 'Kuroshitsuji Harry Potter fanfiction' might involve the two universes colliding during a magical event. The Death Eaters could mistake Sebastian for one of their own due to his dark powers. Meanwhile, the Order of the Phoenix might be suspicious of Ciel's motives in the wizarding world. This could lead to a complex web of alliances and betrayals.
A plot could involve Ciel Phantomhive being magically transported to Hogwarts. Since he's used to a world of mystery and intrigue in his own story, he might quickly adapt to the secrets of Hogwarts. He could form an unlikely alliance with Harry Potter and his friends, and together they face a new threat that combines elements from both universes, like a dark wizard who has made a deal with a demon.
One common theme is the clash of different magical systems. Since Harry Potter has a more 'school - based' magic system and Kuroshitsuji has a more 'contract - based' system, seeing how they interact is a big theme.
A popular one could be 'Merged Realms'. In this fanfic, the Hogwarts and the Phantomhive Manor settings are blended. It often features Harry and Ciel working together to solve a mystery that involves both the wizarding and the demon - filled worlds of their respective stories.
There could also be a kuroshitsuji lemon fanfiction that delves into an alternate universe. In this AU, the characters have different personalities and the story could revolve around a love triangle between Ciel, Sebastian and a new character. The lemon aspect would add a touch of sensuality and passion to the complex relationships in this new world.
Well, perhaps a concept is the power dynamics. If Harry is death in this crossover, he would likely have a great deal of power. In Kuroshitsuji, power is a major theme, especially with Sebastian's demonic powers. Harry's power as death could either clash or complement Sebastian's. This could lead to some really intense and unique storylines where the characters are forced to navigate around these two powerful forces.
There's also 'Merged Realms'. This fanfiction does a great job of integrating the storylines. It starts with Harry Potter accidentally stumbling into the world of Kuroshitsuji. The author has really captured the essence of both universes and created a fascinating story. For example, Harry's magic and the contracts in Kuroshitsuji are combined in a very interesting plot device.
One interesting story could be about Pluto's hidden past in the Kuroshitsuji universe. Maybe it explores how Pluto came to be in the service of the characters in the series. Fanfiction often takes the known elements of the original work and twists them in unique ways.
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.
Sebastian and Snape would be a very interesting pairing. They both have a certain air of mystery and are very skilled in their respective magics. Sebastian could learn from Snape's knowledge of potions, and Snape might be curious about Sebastian's demon abilities.
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.