Well, it could be about Allison's struggle with a mystery illness that may or may not be related to the main PLL plot. Maybe she starts getting sick and her friends suspect it's something more sinister, like someone is poisoning her to keep her quiet about a secret.
Another possibility is that one of the other characters in PLL has a hidden motive. For instance, someone who is jealous of Allison's popularity or someone who she had an argument with in the past. They might have found a way to make her sick without being suspected at first.
Maybe it could be about how Caleb's sickness affects his relationships in the Pretty Little Liars world. For example, how his friends and loved ones react to his illness, and how he copes with it himself while dealing with all the secrets and drama in Rosewood.
The plot might revolve around Aria getting sick as a result of all the stress from the Pretty Little Liars' chaos. Her sickness could be a symbol of how the lies and secrets are taking a toll on her. In the fanfic, we might see her struggle with her illness while also trying to figure out who 'A' is. It could be that her weakened state makes her more vulnerable to 'A's attacks or makes her see things in a different light.
Maybe it's about how Caleb's sickness affects his relationships in the PLL world. For example, how his friends and loved ones react to his illness and take care of him.
In the PLL fanfic, when Caleb gets sick, it could lead to a lot of worry among his friends. They might try to take care of him, like Hanna bringing him soup and making sure he takes his medicine. His illness could also be a distraction from the main mysteries in Rosewood, causing the group to focus more on his recovery for a while.
Well, it could be about Aria dealing with the new experience while also handling the drama in PLL. Maybe she tries to hide it from the other girls at first because she's embarrassed, and then as the story progresses, they find out and support her.
The story would change in several ways. First, the focus would shift a bit to Caleb's health. His friends would be preoccupied with taking care of him. Second, it might slow down the investigation into the usual PLL mysteries as they are distracted. Third, his relationship with Hanna could be put to the test.
Well, perhaps she gets sick because of some sort of environmental factor in Riverdale. It could be related to the polluted river or some toxic gas leak. And this sickness then affects her relationships with Jughead, Archie and Veronica. She might become more withdrawn and they would have to figure out how to help her.
One possible plot in this fanfic could be that Carly gets a really strange and rare illness. Her friends then embark on a journey to find a cure. Along the way, they meet different characters from the iCarly universe who either help or hinder their search. This could include old guest stars or even some of the school bullies who have a change of heart and decide to assist.
Perhaps in the fanfic, Amy Fleming's sickness is a mystery. The story could focus on her trying to figure out what's wrong with her while also dealing with her daily life on the farm. She might have to rely on her horse for comfort during her illness.