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.
It's possible that there's a strange illness going around the village and Naruto goes to the hospital to see if he can offer any help. He has a strong sense of justice and would want to do whatever he can to stop the spread of the illness and make sure everyone gets better.
The hospital setting can enhance their relationship by providing a more intimate and private space. For example, Naruto could be in a private room recovering, and Hinata could visit him often. With fewer distractions compared to their normal busy lives, they can really focus on each other.
Naruto could also be a main character, even if he's not a medical expert. He could be visiting his injured friends in the hospital and accidentally stumble upon a conspiracy. His determination and never - give - up attitude would be useful in uncovering the truth.
I'm not entirely sure as I haven't read that specific fanfiction. But generally, in Naruto fanfictions, a story set in a hospital involving Tsunade could be about medical situations in the Naruto universe. Maybe it has something to do with treating injured ninjas or some sort of mystery related to the hospital's operations.
One common plotline is Naruto's slow recovery from a severe injury. He has to overcome physical pain and weakness, all while trying to keep his spirits up. Another is that he discovers a new power or ability while in the hospital, maybe something related to his chakra that he can only access when he's in a weakened state.
Often, there's the theme of Naruto using his ninja skills for good in the hospital. For example, his ability to sense danger could help in predicting a patient's sudden decline. And his strong willpower could be an inspiration for patients to fight their illnesses. Another theme could be the relationships he forms with the other hospital employees, some might be initially skeptical of him but later come to respect his abilities.
Another theme is friendship. Naruto is a very friendly and outgoing character. In these fanfictions, he usually forms strong bonds with some of the hospital characters. For example, he might become friends with a doctor who is intrigued by his ninja abilities or a patient who he helps in a unique way.
One common plotline is the recovery of a major character. For instance, Naruto might be seriously injured in a battle and his friends gather at the hospital, waiting anxiously for his recovery. Another is the introduction of new medical techniques or characters who are medical experts.