Sakura would be very concerned. She's always been there for Naruto in her own way, and she has medical knowledge, so she would be eager to help him get better.
Iruka - sensei would also be highly concerned. He was Naruto's first teacher who truly believed in him. He would visit Naruto and probably bring some of Naruto's favorite food to cheer him up while he's sick.
A 'naruto has a fever fanfic' could show Naruto's softer side. When he has a fever, he may not be as energetic and strong as usual. His sensei, like Kakashi, might come to visit him and share some words of wisdom. Naruto could also have some fever - induced dreams that are related to his parents or his dreams of becoming Hokage. His friends would be worried and try different ways to lower his fever, like using cool cloths on his forehead. And in the process, the bond between Naruto and his friends could be further strengthened.
The plot could start with Naruto suddenly getting a high fever. His friends notice his strange behavior like being more sluggish than usual. Then, they take him to the hospital. As he's recovering, he could start to remember some forgotten memories related to his parents or some past traumas.
Sasuke would also be concerned, believe it or not. Despite their complex relationship, they are comrades and friends deep down. He might not show it outwardly as much as others, but he would be worried. And of course, Kakashi. As Naruto's teacher, he has a responsibility towards him. He would be using his wisdom to figure out a way to get Naruto back on his feet.
Sakura would be very worried and immediately start taking care of Naruto. She would check his temperature, give him medicine, and make sure he's comfortable. Kakashi might be a bit more laid - back on the outside but would secretly be concerned. He would keep an eye on Naruto's condition and also ensure that the village is still protected. Sasuke, if he's around, might act like he doesn't care much at first, but he would actually be worried. He might even go on a little side - quest to find a special herb or something that could help Naruto get better faster.
Probably Rhodey. He's been Tony's friend for a long time. They've been through a lot together, and Rhodey would be very worried about Tony's well - being.
His closest friends like Kagami would probably be very concerned. They have a strong bond on and off the court.
Iruka - sensei could be an interesting option. He was Naruto's first teacher and has always had a soft spot for him. He would bring the warmth and comfort of a father figure. He might not have the advanced medical skills like Sakura, but he would stay by Naruto's side, tell him stories to keep his spirits up, and do whatever he can to help Naruto get better.
Sakura would probably be the first to jump into action. As a medical ninja, she'd immediately start examining Naruto and try to figure out the cause of the fever. She might be worried but also very focused on treating him.
Steve Rogers would probably be one of the most concerned. His sense of duty and protectiveness would make him take the 'get help' situation very seriously. He'd be worried about the safety of those in need.
Sakura would be a likely candidate. She's a medical ninja, and she has always cared about Naruto. She would use her medical skills to try and stabilize him.