Sakura's teammates from her original team could be likely characters. They would be worried about her and try to figure out a way to get her out. There could also be some local Kirigakure ninja who are curious about this outsider in their prison and interact with Sakura in different ways, either as friends or foes.
One possible plot could be that Sakura is wrongly imprisoned in Kirigakure prison. She might have to use her medical and combat skills to survive among the dangerous inmates. Maybe she is there on a mission to find a hidden secret or rescue someone else who is wrongly accused.
Natsume would probably be one of the main characters to appear. Their relationship was quite significant when they were younger, and it could continue to evolve as they grow up. Hotaru might also be there, as she was Mikan's good friend. And there could be new characters introduced, like colleagues if the fanfiction focuses on her career, or new friends she makes in her adult life.
Sasuke would be a likely character to appear. Since he and Naruto have a deep connection, Sasuke might cross paths with Naruto during his wanderings, either as an ally or an antagonist depending on the plot.
Naruto is a very likely character. Given his rebellious nature and his history of being an outcast in some ways, it's easy to imagine a scenario where he gets caught up in something that lands him in prison. His determination to break free and his loyalty to his friends would make for an interesting story.
There could be the prison guards. Some might be cruel and strict, while others could be more sympathetic. Also, other inmates. Maybe an older, experienced inmate who becomes a mentor - like figure to Sakura.
Since the title only mentions Sakura, she's likely to be one of the main characters. But there could be others, like the prison guards or fellow inmates. Maybe there's a character who befriends Sakura or an antagonist who makes her life difficult in the prison.
Obviously, Paul himself is the central character. His brother Reggie might also appear as he could be concerned about Paul's condition and offer support. Ash could be in it too. He might be someone who initially doesn't get along with Paul but then shows compassion because of his illness. And of course, Paul's Pokemon would be there, like his Torterra or Electivire.
Sonic himself, of course. And his best friend Tails is very likely to be there. Tails is always by Sonic's side and would surely be there when Sonic is sick.
Hermione would likely be one of the characters. She's Harry's best friend and would probably try to find him and help him out. Ron might also be there, as he's always been by Harry's side. Dumbledore could be another character. He might be trying to keep an eye on Harry from afar, or perhaps he has a plan for Harry's new situation. Hagrid would also be a likely character. He has a soft spot for Harry and would want to make sure he's okay. And of course, the Dursleys themselves might make appearances, either to continue their mean - spirited ways or to have a change of heart.
Marinette and Adrien are also very likely to be in it. Since they are the civilian identities of Ladybug and Chat Noir respectively, they could be at the beach with their friends, like Alya and Nino. Maybe Alya could be trying to get some juicy photos for the Ladyblog while at the beach.