Another plot might involve his love interest seeing this new side of him and falling for him even more. For example, they could find his childish pouts and demands adorable while nursing him back to health. This can strengthen their relationship in the fanfiction. Also, it could lead to some really sweet and romantic moments between them.
A common plot could be his friends trying to take care of him while being amused by his child - like behavior. They might struggle to get him to take his medicine because he acts like a stubborn child.
Sometimes, the plot could revolve around Shinichi being in a situation where his child - like sick behavior gets him into trouble. Maybe he accidentally reveals a secret or does something embarrassing while in that state. And then the story could be about how he and his friends deal with the aftermath of that situation.
In such fanfictions, Shinichi acting like a child when sick could be a form of character development. Maybe he has suppressed his more carefree and child - like nature due to his detective work. The sickness acts as a trigger to bring out this side. For example, he might become more clingy or whiny, which is a contrast to his usually independent and intelligent self.
Often, there's a plot where the sickness is some sort of mysterious ailment that no one really understands at first. So, the characters like Sans or Papyrus might try to use their own unique ways to figure out what's wrong with Frisk. Papyrus could try to 'doctor' Frisk up with his 'amazing' but not always accurate medical knowledge.
One common plot could be her fellow Jedi or friends trying to find a cure. For example, Anakin might search for rare herbs to heal her. Another plot could involve her having a vision during her illness that guides her or her friends on a future mission. Also, it could be about how she copes with being sick while still feeling the need to contribute to the Jedi Order's activities.
Some fanfictions might have a plot where Sebastian's sickness makes him more vulnerable to his demonic urges. Normally, he has great control, but being ill could weaken that. This could lead to dangerous situations in the manor. There may also be a plot where a mysterious character appears, claiming to have a cure for Sebastian, but with a price that Ciel has to pay, which then creates a moral dilemma for Ciel.
Another plot could involve Tails trying to hide his sickness at first because he doesn't want to worry his friends. But as it gets worse, he can't hide it anymore. His friends then feel guilty for not noticing earlier and they all come together to take care of him and find a remedy.
One plot could be Naruto getting a mysterious illness that no one in the village can figure out. His friends, like Sakura and Sasuke, start to search for a cure. They might go on a journey to find a rare herb or seek the help of an old sage. Another could be that the illness is a result of a new enemy's jutsu. As he gets sicker, the village starts to worry about their future without their hero.
One possible plot could be that Naruto's friends have to take on his missions while he is sick. They struggle at first because they realize how much Naruto usually does. Another could be that Naruto tries to hide his sickness from everyone, but his body keeps giving him away, like fainting during training. And a third plot might be that a new enemy takes advantage of Naruto's weakened state to attack the village.
One possible plot could be that Batman gets a mysterious illness that not even the Batcave's high - tech medical equipment can diagnose. As he gets weaker, Gotham's criminals start to become bolder. The Bat - family has to work overtime to keep the city in check while also trying to find a cure for Batman.
One common theme is friendship. When Mario gets sick, his friends always come to his aid, showing the strong bonds between them.
A great plot could be that Hiccup's sickness is caused by a mysterious dragon - related illness that no one has seen before. The riders then have to embark on a journey to find a cure. Along the way, they encounter new dragons and face various challenges. Hiccup, despite being sick, uses his wits to help them out.