For main characters, Severus Snape could be in a 'Harry Potter mental hospital' fanfiction. Even though he sacrificed himself, he might have left some mental trauma behind. His presence in the hospital could be a mystery as he was thought to be dead. Another could be Madam Pomfrey. She could be in charge of the medical side of things in the mental hospital, using her years of experience at Hogwarts to try and heal the wizards. And Fleur Delacour. She might be there because she was affected by some dark magic during the Triwizard Tournament, and her story in the mental hospital could be about her regaining her confidence and mental strength.
One main character could be Draco Malfoy. After the war, he might be suffering from guilt and depression, leading him to the mental hospital. He could be a complex character, struggling between his old Slytherin values and the new understanding of life after the war. Ginny Weasley could also be a main character. She might be visiting the mental hospital as part of her work in the wizarding community to help those in need, and she forms a bond with some of the patients, especially if Harry is among them. And then there's Neville Longbottom. He could be working at the mental hospital, using his skills with plants to develop new therapies for the patients.
Obviously, Harry Potter himself could be one of the main characters. Maybe he's been through so much during the fight against Voldemort that he starts to have mental health issues and ends up in the mental hospital. Hermione would also be a likely main character as she would be trying to find a way to cure Harry using her vast knowledge of magic. Another could be Luna Lovegood. Her unique perspective on the world might make her either a patient who is wrongly diagnosed or a healer who has her own unorthodox methods of treating the mentally ill wizards.
Sakura could also be a main character. As a medic - ninja, she might be involved in Naruto's treatment in the mental hospital. She could bring her own knowledge and her relationship with Naruto into play, adding depth to the story.
One possible plot could be that some of the Hogwarts students end up in a mental hospital due to the trauma of the wizarding wars. They form a support group within the hospital, using magic secretly to heal each other's mental scars. For example, Harry Potter could be haunted by the deaths he witnessed and struggle with PTSD in the hospital. Hermione might use her knowledge to research magical cures for mental illness.
Obviously, Harry, Hermione, and Ron could be main characters. They often get into trouble and end up in the Hospital Wing. They could be there to visit an injured friend or because they themselves are injured. Neville could also be a main character, maybe he discovers some new plants that have healing properties in the Hospital Wing.
Perhaps a powerful monster in disguise as a doctor or staff member. They could be using the hospital's resources to create a portal to the underworld or something equally dangerous, and Percy has to stop them. They would be constantly trying to drive Percy crazy or keep him from interfering with their plans.
Lucy might also be a suitable main character. Her kind heart would lead her to try and help the other patients. She could use her celestial magic in unique ways to communicate with those who seem lost in their own minds. Maybe she gets trapped in the mental hospital while looking for a lost Celestial Spirit item and has to find a way to escape while also helping those in need.
Ron Weasley can also play an important role. He might be the one who accidentally causes an accident that sends someone to the hospital, and then feels really guilty about it. So he spends a lot of time in the hospital wing trying to make it up to the injured person, while also getting into some comical situations with the other patients and the Healers.
The hospital staff also feature prominently. Nurses and doctors play important roles. A strict but kind - hearted doctor could be a main character, who is dedicated to finding new ways to treat the patients in Sakura Mental Hospital. The fanfiction might show their daily struggles and triumphs in dealing with the patients.
It's possible that the main antagonist is a new mythological creature that has the ability to manipulate minds. It has lured Percy and other demigods to the mental hospital and is slowly driving them insane in order to feed off their fear and chaos energy.
Obviously, the Avengers themselves would be main characters. Iron Man could be the one who funds the high - tech medical equipment in the hospital. Thor might have a hard time adjusting to the human medical procedures. And Black Widow could be using her espionage skills to find out if there are any threats in the hospital.
Another character could be Kakashi. He might be the one who doubts the decision to send Naruto to the mental hospital. He could secretly visit Naruto and try to train him in mental fortitude techniques so that he can get out of the hospital sooner.