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.
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.
Well, one possible plot could be that Naruto has faced so many traumas during his battles and the loss of his friends that it finally takes a toll on his mental health, leading to him being sent to a mental hospital. In the hospital, he might have to deal with his inner demons, like the Nine - Tails' influence on his psyche, and try to find a way to heal.
Hinata would also be very involved. She has deep feelings for Naruto. She might be at his bedside in the hospital, praying for his recovery. After his death, she would be heartbroken and might have to find a new purpose in life without Naruto.
Another character could be Kakashi. As Naruto's former teacher, he has a lot of wisdom. When Naruto tells him about the future he's seen, Kakashi could offer strategic advice on how to prevent the negative events from occurring. He could also help Naruto understand the possible consequences of their actions to change the future.
There could be a kunoichi who is a master of stealth and espionage. She might be the first to discover Naruto's arrival in the kunoichi world. Another main character could be a kunoichi with a strong sense of justice, similar to Naruto. She would be the one to advocate for Naruto when others are against him. And perhaps there's a kunoichi who has a special connection to an ancient power in this world, and she and Naruto's Juubi power somehow interact in an unexpected way throughout the story.
There could be some original Asian - inspired characters. For example, a young martial artist from a small village in China who Naruto takes under his wing. This character could be eager to learn from Naruto and would be one of the main focuses as he grows under Naruto's guidance.
The orphans themselves would be main characters. Each one could have their own backstory and personality. Some might be shy, while others could be more outgoing. They would be the ones directly affected by Naruto's care and teachings.
His teammates, Sasuke and Sakura, would surely visit him. They are his closest comrades, and they would be worried about his condition.
Tsunade for sure. As the Hokage and also a medical expert, she would be furious about Naruto escaping the hospital. She might send some ninjas to bring him back.
Sasuke is often a main character in such fanfictions. He could be Naruto's right - hand man, using his skills and intelligence to support Naruto's rule. Their relationship might be more complex, with Sasuke having to come to terms with Naruto's new position of power.