Well, you could write a fanfiction where the characters go on a road trip together. This would give room for a lot of character development and interesting interactions. There could be conflicts like getting lost or running out of supplies. Another idea is to have a character deal with a family issue that affects their relationships in the series. And perhaps a plot where the characters participate in a school competition that has high stakes and they have to work together to win.
One idea could be Finn getting a bad case of the flu right before a big performance. His friends in Glee club would then have to figure out how to cover for him or help him get better in time. Another could be that Finn has an ongoing illness that he's been hiding from the club, and it finally starts to affect his ability to participate fully. Maybe he faints during a rehearsal, and then everyone rallies around him to support him through his sickness.
Another plot could involve Five getting sick due to some side - effect of his powers overusing them in a short period. As he gets sicker, he starts to lose his memories of different timelines he has visited. The story could then focus on his journey to regain those memories with the help of his siblings.
A possible plot could be that Levy's sickness is a side - effect of overusing her magic while trying to save the guild from a great danger. Her fellow Fairy Tail members take turns to take care of her and in the process, they all learn more about the importance of balance in using magic.
One idea could be that Percy's sneeze accidentally triggers some sort of magical reaction. For example, it could cause a mini - tsunami in the middle of Camp Half - Blood because his powers are all jumbled up due to his sickness. Another plot could involve Percy sneezing and accidentally revealing his location to some monsters who are on the hunt for him while he's weakened by the illness. Maybe his friends have to figure out a way to keep him hidden and also cure his sickness at the same time.
One plot idea could be Sebastian falling ill due to a curse placed on him by a rival demon. He would struggle to maintain his usual composed and efficient self while still trying to serve Ciel. Another could be that he gets a mysterious illness that makes him lose his demonic powers temporarily, and he has to rely on human means and Ciel's help to recover. Maybe he gets sick from consuming something in the human world that his demon body is not used to, like a poisoned food item that affects him in strange ways.
One idea could be that her illness is a result of some forbidden jutsu she tried to master. Her friends in the village then have to find a rare herb to cure her. They embark on a dangerous journey, facing various enemies and challenges along the way.
A really interesting plot could be that Kendall has a chronic illness that he's been hiding from his fans and the rest of Big Time Rush. But as the symptoms start to get worse, he has no choice but to take a break from the band. His fans start to notice his absence and rumors spread. A young fan, who has the same illness, starts an online campaign to show Kendall that he's not alone and that they support him no matter what.
One idea could be that while Henry is sick, his friends at Junk N' Stuff try to find a special cure using their wacky inventions. Maybe they accidentally create a super - powered medicine that gives Henry temporary super - abilities along with curing his illness.
One idea could be that Tanya tries to use her unique abilities to heal Bella. Since Tanya is often depicted as having certain powers in related stories, this could be a central plot point where she battles against Bella's unknown illness.
Perhaps a story where a character loses their love due to a misunderstanding, but then through a series of events, they are brought back together. It could involve self - discovery for both characters, realizing that their love was always meant to be, and they have to love and love again after learning from their past mistakes.