Another idea is a crossover with 'Fullmetal Alchemist'. Ciel could be interested in the alchemy in that world and perhaps try to use it for his own purposes. Sebastian would be there to either help or hinder depending on his own motives.
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.
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, 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.
One interesting concept could be a crossover with other Victorian - era related works. It might bring in different characters and storylines from that time period, adding more depth to the Kuroshitsuji universe. For example, a crossover with Sherlock Holmes - like characters could lead to some exciting detective - related plots within the Kuroshitsuji world.
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.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
A Thundercats and Star Wars crossover fanfiction would be really cool. The Thundercats could be in the Star Wars universe, and their unique powers and weapons could be a great addition. Maybe the Thundercats could help the Jedi or the Rebels in their fight against the Sith or the Empire. For instance, Cheetara's speed could be useful in battles, and Tygra's illusions could be used to deceive the enemy.
Begin with a strong hook. It could be a situation that immediately throws the characters from both universes together in an unexpected way. For example, if it's a crossover with 'My Hero Academia', start with Ciel and Sebastian suddenly appearing in the middle of a villain attack at UA High School. This will draw the readers in and make them want to read more.
A Digimon and Sailor Moon crossover is an interesting concept. The Digimon could help the Sailor Scouts in their battles against the Dark Kingdom. The magical powers of the Sailor Scouts combined with the digital powers of the Digimon would create a really unique and exciting story. And a Digimon and Avatar: The Last Airbender crossover. The Digimon could be part of the different nations, and they could use their abilities to help Aang and his friends in their quest to restore balance.
One idea could be an Atreus crossover with the world of Harry Potter. Atreus could be a new student at Hogwarts, bringing his own unique skills and knowledge from his Norse mythology background. He might struggle to adapt to the magic of the wizarding world while also showing off his own combat and survival abilities.