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.
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.
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.
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.
Perhaps in the fanfiction, Mikasa believes that Eren has gone too far with his plans and she betrays him to stop him from causing more destruction. She might think that his actions are endangering not only their friends but also the entire world. So, she makes the difficult decision to betray him in an attempt to save everyone else.
One common plotline could be the other turtles trying to take care of Leo. They might struggle to figure out what's wrong with him at first. For example, Donatello would use his scientific knowledge to run tests. Another plotline could involve Leo being stubborn about accepting help while he's sick, thinking he can still lead and protect the team even in his weakened state.
A possible plotline is that Ash gets a mysterious illness that affects his ability to communicate with his Pokémon. This causes chaos among his team as they don't understand why he's acting differently. Meanwhile, Team Rocket might try to take advantage of Ash's weakened state but end up having a change of heart when they see how sick he really is.
A simple plotline could be that Bella's sickness is just a normal flu, but it makes her realize how much she depends on those around her. She might have a moment of self - reflection while lying in bed sick. She could also receive some unexpected help from a stranger who turns out to be an important character later in the story.
Perhaps Stiles is killed as a sacrifice in a dark ritual. His death could be part of a much larger conspiracy. His friends might start seeing strange visions of him after his death, which they initially think are just their minds playing tricks on them, but later realize are actually Stiles trying to communicate from the afterlife to help them stop the forces behind his death.
One possible plotline could be that Harry gets hurt during a Quidditch match. Maybe a Bludger hits him hard and he has to deal with a serious injury like a broken bone. He then has to struggle with not being able to play for a while and the fear of not being as good when he recovers.