Perhaps in the fanfiction, Tobirama's sickness makes him weak and he starts to reflect on his life and the choices he has made. He could have conversations with his loved ones where he reveals his regrets or hopes. This introspective side of him due to the sickness could add depth to his character. Also, his sickness might be a catalyst for changes in the village he has helped build, as others step up in his absence.
Perhaps Mikasa gets sick right before a big battle or important mission. She is determined to still participate, but her body won't allow it. This causes her to feel frustrated and guilty. Her comrades then have to step up in her absence, and she has to watch from the sidelines as they face danger. Meanwhile, she focuses on getting better so she can rejoin them.
One possible plotline could be Robin trying to hide their illness from their comrades at first. They might be worried about appearing weak or burdening the team. As the sickness progresses, their friends start to notice the changes in Robin's behavior and performance. Eventually, they find out and rally around to take care of Robin, bringing in healers and trying different remedies.
Another plot could involve Naruto being revived due to the strong will of the people. In the Naruto universe, the will of the people can have a strong influence. So, all the people who Naruto has helped and inspired over the years unconsciously create a kind of spiritual energy. This energy is noticed by a sage or some other powerful being who uses it to bring Naruto back. Once revived, Naruto uses his new - found spiritual connection with the people to fight an even greater evil.
One possible plotline could be that Loki meets a girl in a different realm. Maybe she's a powerful sorceress who initially challenges him. As they cross paths more often, they start to see each other's true selves. Loki, who is used to being alone and distrusted, slowly opens up to her. Their relationship could be filled with banter and magical adventures as they face threats to their realms together.
One possible plotline could be that Naruto is granted wings as a result of some ancient and powerful jutsu he accidentally activates. With the wings, he could gain new abilities like enhanced speed in the air and new combat techniques. For example, he might be able to use the wind generated by his flying to create more powerful Rasengan attacks. Another plotline could be that the wings are a gift from a mysterious being who sees Naruto's pure heart and wants to help him in his fight against evil. He then has to learn to control the wings and use them to his advantage in battles against stronger opponents.
A plot could involve Sarada getting hurt in a trap set by a villain. The villain might be targeting the new generation of ninja in the village. As she gets injured, it sets off a chain of events where her friends start to investigate the incident. Sarada, while hurt, could provide some crucial clues from what she saw before getting injured. This would not only be about her physical injury but also about her mental strength as she tries to contribute to the solution while in pain.
In some fanfictions, the villains might take advantage of his sickness. They could try to attack him when he's weak. But of course, his friends will be there to protect him. His sick days could also be a time for him to reflect on his life as a hero - in - training. He might think about his relationships with his classmates and how he can improve himself once he gets better.
One common plotline could be Deku trying to hide his sickness from his friends at U.A. High so as not to worry them. He might keep pushing himself during training, which could lead to him collapsing eventually. Another could be his friends finding out about his illness and rallying around him to help him get better, like bringing him home - made soup and study notes so he doesn't fall behind in class.
One reason could be that he was exposed to some kind of harmful chakra or energy in the village. Maybe some leftover dark jutsu residue affected his weak baby body.
In fanfiction, one possible reason could be that he was exposed to some kind of harmful chakra. Maybe an enemy sent a weak but insidious chakra wave towards him when he was just an infant, which gradually made him sick. Another reason might be related to the poor living conditions he was in. Since he was alone as an infant without proper care, he could have caught a disease easily from the dirty environment around him.