Well, Naruto for sure. His friends from Team 7 are highly likely to be in it. But also, characters like Tsunade. As the Hokage and a great medical ninja, she could be consulted if Naruto's illness is really serious. Jiraiya might also make an appearance in Naruto's sick day fanfiction, perhaps reminiscing about his own times when he was sick as a young ninja.
One common plotline is that Naruto's friends take care of him. For example, Sakura might use her medical skills to nurse him back to health. Another could be Naruto having flashbacks or deep thoughts while sick, perhaps about his lonely childhood. And sometimes, enemies might take advantage of his weakened state, leading to some tense situations where his friends have to protect him.
Toriel is very likely to be involved. She has a motherly nature and would surely be concerned about Frisk's health. She would probably be at Frisk's bedside, offering comfort and warm food.
Well, Mercedes Jones would be a key character. She has a strong sense of empathy and would use her voice to bring comfort to the baby and the family. Kurt Hummel is also very likely. His fashionable and caring nature would make him be there for the baby. Puck could also be involved, perhaps using his tough - guy exterior to shield the family from any trouble in the hospital environment.
Iruka and Mizuki are also possible characters for role reversal in fanfiction. Iruka could be the one with some hidden, not - so - good motives, while Mizuki could be the kind - hearted teacher figure. This would turn the early Naruto story on its head, as Iruka was Naruto's first positive influence in the original series. The new dynamic between Naruto and these two reversed characters would open up a whole new set of story possibilities.
Obviously Naruto himself. Then his parents, Minato and Kushina. His wife Hinata and their children would also be part of it. Jiraiya could also be involved as he was like a father figure to Naruto. And maybe some of the remaining Uzumaki clan members if they are still alive in the fanfiction. Also, characters like Iruka, who was an important person in Naruto's early life, could be there to witness the family reunion.
Naruto himself is very likely to be involved. He's the main character of his series and his unique abilities like the Rasengan and shadow clones would make for great interactions. Sasuke is also a top candidate. His Sharingan and his dark past could create interesting storylines when combined with hero concepts. Sakura, with her medical ninjutsu and strength, could also be involved, perhaps teaming up with a hero who has a similar healing - based power.
Obviously, Naruto himself is central. His family like Hinata and Boruto would be very likely to be involved. Sasuke is also a key character as Naruto's friend and ally. Kakashi, being Naruto's former teacher, could also play a role. Shikamaru, as Naruto's advisor, would be important in any political or strategic situations. And then there are the other members of the Konoha 11, like Sakura who could be involved in medical support or combat situations if needed.
Well, Danny Phantom is obviously the main character. Since it's about his sick day, the story would likely revolve around him.
Sasuke is very likely to be involved. Since he's Naruto's rival and friend, he might be there to witness the meeting of different Narutos. He could be confused at first, but then he'd probably try to figure out how this strange phenomenon occurred. He might also get involved in battles or discussions that the Narutos are having.
There could be new characters specific to the village where he becomes Kamikage. For example, some powerful shinobi clans within that village. Their leaders would be important as they could be Naruto's allies or opponents. Also, the previous Kamikage's advisors might still be around and play a role in guiding or challenging Naruto. Maybe an old sage - like figure who has deep knowledge of the village's secrets and helps Naruto understand and respect the village's heritage.