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's also 'Magic and Contracts'. This fanfic delves deep into how the magic from Harry Potter's world would interact with the contract - based magic in Kuroshitsuji. It shows how characters like Harry might be affected by the rules of the Kuroshitsuji magic system and vice versa.
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.
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 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.
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.
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.
One great 'Harry Potter brother fanfic' is 'The Lost Potter'. It offers a unique take on Harry having an unknown brother. The story is filled with adventure as they discover their connection and face the challenges of the wizarding world together.
One great Griffin Harry Potter fanfic is 'The Griffin's Bond'. It focuses on a unique magical bond between Harry and a griffin character. The story is full of adventure as they face dark wizards together.
One popular fanfic might be 'The Meeting of Magical Worlds' which could potentially bring together the elements of Harry Potter and Hela in a unique way. It might explore how Harry's magic would interact with Hela's powers.
Well, there's a fanfic called 'The Potter Emperor's Reign'. In this story, Harry Potter uses his unique skills and knowledge to become an emperor in the wizarding world. It has a really interesting plot where he reforms the Hogwarts curriculum and also the Ministry of Magic. Then there's 'Emperor Potter: A New Era' which shows how Harry, as an emperor, brings about peace between different magical species. Also, 'Harry Potter: Imperial Ascendancy' is quite popular, with a focus on his battles against the dark forces as an all - powerful emperor.
There's also a fanfic series that focuses on Harry Potter as a metaphor for the journey of faith. In these stories, Harry's battles against Voldemort are paralleled to a Christian's struggle against evil. It delves deep into how the magic in the Potterverse can be seen in a new light when related to religious beliefs.