In this fanfic, Ciel could potentially be sorted into a Hogwarts house, say Slytherin because of his cunning nature. Then he would have to navigate the politics within the house while also exploring the secrets of the castle. He could also get involved in some kind of mystery that threatens Hogwarts and use his unique skills to help solve it.
I'm not sure specifically as there could be many different versions of such a fanfic. But generally, it might involve Ciel's journey to heaven, perhaps his experiences, the people or entities he meets there, and how he copes with this new environment.
Ciel is surely the central figure. However, there could be other characters from the original source material that have also passed on and are in heaven. Additionally, there may be original characters created for the fanfic, such as a special angel who has a particular connection with Ciel. There might also be antagonists in the form of fallen angels or some sort of heavenly bureaucrats who oppose Ciel for some reason.
Well, it could be that Percy has a hard time adjusting to the wizarding world at first. But then he meets a mentor figure who helps him understand his place in Hogwarts and he starts to thrive. He might even join the Quidditch team and become a star player.
Well, it could be about Naruto arriving at Hogwarts and being confused by all the magic there. Maybe he tries to use his ninja skills in a world full of wizards and witches.
One popular 'bts goes to hogwarts fanfic' is 'The Magic of BTS at Hogwarts'. It's really interesting as it combines the unique personalities of BTS members with the magical world of Hogwarts. For example, Jin might be in the Gryffindor house, showing his brave side while also being the 'worldwide handsome' in the wizarding world.
The sorting ceremony is often exciting. Imagining which Hogwarts house each BTS member will be sorted into is really cool. Like, will RM be in Ravenclaw for his intelligence? Another exciting part is when they face magical challenges together, just like how they face difficulties as a group in real life.
One possibility could be that Harry discovers he has some other magical destiny outside of Hogwarts. Maybe he is taken in by a different group of wizards who teach him their own brand of magic in a hidden location. They might believe Hogwarts is too mainstream or that Harry has a special power that needs to be nurtured away from the school.