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.
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.
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.
First, understand the characters from both universes well. Know Ciel's and Sebastian's personalities from 'Kuroshitsuji' and the traits of characters like Harry, Hermione, and Ron from 'Harry Potter'. Then, come up with a unique plot that combines the elements of both worlds. For example, a plot where the Philosopher's Stone is being hunted by demons from the 'Kuroshitsuji' world.
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.
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.
Fanfiction.net is another good place. There are many talented writers who post their Harry Potter Kuroshitsuji fan creations there. However, the search functionality might be a bit less refined compared to AO3. But if you dig a bit, you can still find some great stories.
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 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.