Obviously, his old friends like James Potter or Remus Lupin could come to visit him in the hospital. They would bring some of their old Hogwarts memories and cheer him up. Then there could be the hospital staff, like a strict but kind - hearted matron who takes care of Sirius. Also, other patients in the hospital might interact with him, such as an old wizard who has some valuable advice for Sirius.
Harry Potter could be one of the characters. He would be worried about Sirius and would visit him often. He might bring some of his schoolwork to show Sirius and seek his advice. Hermione could also come along. She would probably be researching in the hospital library to find a cure for Sirius' illness. And don't forget Dumbledore. He might drop by to have some deep and mysterious conversations with Sirius about the future of the wizarding world.
A merman could be one. He might be curious about the Black Sea Reaper and try to make friends or, on the contrary, see it as a threat to his underwater territory. Another could be a sea witch. She could have her own motives, either to help or to hinder the Black Sea Reaper. A young sea turtle could also be a character. It could look up to the Black Sea Reaper as a hero and follow it around on its adventures.
Tsunade would definitely be one. As the most renowned medical ninja in the village, she could either be Naruto's mentor or a colleague. They could exchange medical knowledge and experiences.
There could be new characters as well, like a devil mentor figure who teaches Percy about his devil heritage. This character could be ancient and have a lot of knowledge about the devil realm. Or there could be an angelic character who is initially an enemy but later realizes Percy is not fully evil and they end up forming an unlikely alliance.
Naruto might interact with Eostia's warriors. They could train together, Naruto showing them his ninja combat skills and them teaching him about Eostia - style combat. There could also be some mysterious characters in Eostia, like an old hermit who has some knowledge about Naruto's sudden appearance in their world and could guide him on his journey in Eostia.
Sasuke could be one. He might be intrigued or even challenged by Naruto's new wild ways. Their rivalry could take on a new dimension as Sasuke tries to understand what has changed Naruto so much.
Maybe some of Sirius' old schoolmates that he hasn't seen in a long time could be introduced. They could come to visit Sirius and Harry and bring with them new stories and magic. Another potential character could be a mysterious neighbor who is also a wizard but has been in hiding. This neighbor could either be a friend or a foe, adding an element of mystery to the story.
Well, in addition to Sakura, there might be a strict but fair psychiatrist. He could be a key character as he tries to diagnose and treat Sakura. There could also be a group of patients that Sakura befriends. For instance, a young girl who is in the hospital due to severe anxiety and an older man who has hallucinations. These characters could interact with Sakura in different ways, some being helpful, others being a hindrance to her progress in the mental hospital.
In the 'Tony Stark Reborn as a Cat' fanfiction, Rhodey could be an important character. He'd probably be really surprised to see his friend as a cat but would still try to support him. There might also be some villains who encounter cat - Tony. They might underestimate him at first, thinking he's just an ordinary cat, but then be in for a big surprise when he uses his Stark - like wits to outsmart them. Additionally, other superheroes like Captain America or Thor could also cross paths with cat - Tony and have some really interesting interactions.
He could interact with his old friends at Goode, like Rachel. She has a special connection to the demigod world, and their relationship could be further explored. Also, there might be new teachers or students that Percy meets. Maybe a new student who is actually a demigod in hiding too, and they start to figure out each other's secrets.
Obviously, Buffy would be a major character he'd interact with. They could start off as wary of each other but then grow to respect one another's fighting skills. Giles too, as he has a wealth of knowledge about the supernatural, and he and Harry could have some interesting discussions about different types of magic.