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.
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.
Another potential main antagonist could be a force from Harry's world that has followed him into the Kuroshitsuji crossover. This could be some sort of entity that doesn't want Harry interfering with the natural order of the Kuroshitsuji universe. It could be a being that has its own motives for keeping death and the afterlife in a certain state and sees Harry's actions as a threat, and so it actively tries to stop him.
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.
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 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.
Sebastian is often a popular choice. His demonic nature and his loyalty (in a rather twisted way) to Ciel make him a very interesting character to cross over. He can easily interact with characters from other dark or supernatural works, and his powers and suave personality can add a lot to a crossover story.
One common theme is the clash of different power structures. In 'ao no exorcist', there are exorcist powers and demon - slaying swords, while in 'kuroshitsuji' there are demon contracts. When these two worlds cross, seeing how the characters with different power sources interact is a big theme.
In terms of plot, these fanfictions might explore different scenarios. For example, a male oc could be inserted into the main storyline, perhaps becoming an ally or a rival to the main characters. This can lead to interesting twists and turns as they interact with Sebastian and Ciel.