Akira is the main character for sure. There could also be his family members waiting for him at home, like his parents or siblings.
Aside from Akira, there might be a pet that he left at home. The pet could be really excited to see Akira again. And there could be neighbors who remember Akira and are glad to see him come back. Maybe there's also an old mentor or teacher in the town where he lives who has some words of wisdom for him when he returns.
It could be about Akira's journey home. Maybe he faces various obstacles on the way, like bad weather or difficult terrain. He might also meet some interesting characters during his trip.
Since I haven't read it, it's hard to say for sure. But it could feature the main characters from the original Akira like Kaneda or Tetsuo. They might be the ones being interrogated or perhaps the ones doing the interrogation, depending on how the fanfic is written.
There could be other Hogwarts students. For example, Neville could be there, exploring the different plants and herbs that are associated with Beltane. Luna Lovegood might also make an appearance, with her unique perspective on the magical aspects of the festival. And perhaps some local wizards and witches from the area where the Beltane celebration is taking place.
There could be angels, like Gabriel or Michael. They might play a role in guiding Cisl through heaven or testing Cisl's worthiness.
He could interact with Harry Potter, Ron Weasley and Hermione Granger. They might be curious about Ciel and his different upbringing.
It could be some powerful demon lords in hell. They might see Naruto as an intruder and try to stop him from whatever his goal is.
He could meet the employees of Taco Bell. They might be really nice and helpful, or they could be a bit odd.
One possible theme could be the concept of 'returning' or 'coming back'. It shows Jack's journey back to his place of comfort, which could symbolize a return to safety or familiarity.
A powerful dragon that guards the school's most precious treasures could be one of the characters. Also, there might be a group of elves who are experts in nature magic and they could be either friends or rivals to Sakura. Another possibility is a half - human, half - phoenix character who has the ability to heal with fire.
Obviously, Sakura is one of the main characters. There could also be a friendly monster teacher to guide her in the new school.