Well, in some fanfiction, Naruto could be in the hospital because of a new and strange illness that no one has seen before. The story could focus on the doctors in the ninja world trying to figure out a cure. And Naruto being Naruto, he might be cracking jokes with the nurses and other patients, spreading his positive energy even while sick.
In fanfiction about Naruto in the hospital, he could be having some sort of spiritual or mental breakdown that's manifested physically, and the hospital is as much a place for his body to heal as it is for his mind. His friends might gather around, sharing stories and memories to help him regain his mental strength. And perhaps there's a mysterious figure who visits him at night, offering cryptic advice about his future as a ninja.
In the fanfic, Naruto might be in the hospital because of some strange illness that no one has seen before. The doctors in the hospital are trying all kinds of methods to cure him, and he might be feeling really frustrated lying in the bed all day. He could be thinking about all the things he wants to do once he gets better, like going on new missions or training harder.
In fanfiction, when Naruto goes to the hospital, he could discover a new medical jutsu being developed there. He might get involved in the process, either by accident or because the doctors need his help. Since he has so much chakra, it could be useful for testing or something related to the new jutsu.
In fanfiction, Percy could be in the hospital because he was captured by an enemy and tortured. Now he's there being healed by the camp's healers or some advanced demigod medical technology. Another possibility is that he's having some sort of identity crisis related to his demigod status and it's causing him physical and mental stress, so he's in the hospital for both medical and psychological help.
Ash and Serena might meet some new and strange Pokemon in the hospital's garden. These Pokemon could have unique abilities that help in their recovery. For example, a small, furry Pokemon that can emit a soothing glow.
In the 'pokemon ash in the hospital fanfiction', it could be that Ash got poisoned during an encounter with a new and dangerous Pokémon. While in the hospital, he starts to have strange dreams about a powerful Pokémon he has never seen before. His friends come to visit him, like Misty and Brock. Misty might scold him a bit for being too reckless but also show her concern. Brock could be using his knowledge of herbs to see if there are any that can help Ash's recovery. And all the while, Ash is thinking about getting back on his journey as soon as possible.
Maybe Harry is injured during a Quidditch match and has to be rushed to the hospital. There he might meet some interesting new characters, like a healer with a mysterious past.
In the fanfiction, Danny might be in the hospital because of his ghost - fighting injuries. Maybe he has to hide his ghost powers from the doctors while trying to recover.
Light could meet a mysterious patient in the hospital who seems to know more about his Kira identity than he should. This would create a tense situation as Light tries to figure out how much the patient really knows and whether he needs to use the Death Note on this new threat.
In this fanfiction, maybe the Pokemon gets injured during its trip and has to be treated in the hospital. For example, it could have been in a battle with a wild Pokemon and got hurt.
In this fanfiction, perhaps May and Drew are both in the hospital after getting caught in a strange Pokemon - related incident. They could start to realize their feelings for each other while recovering. Drew might share his dreams with May, and May could open up about her insecurities, strengthening their bond.