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.
Well, perhaps a plotline where Ash discovers that being the chosen one means he has a connection to an ancient Pokémon prophecy. This prophecy might involve saving the Pokémon world from a looming disaster. Along the way, he might gain new Pokémon partners that are specifically destined to help him in this grand task. He could also face challenges from other trainers who are jealous of his chosen - one status.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide range of wild and creative ideas, but this one might be quite rare or not in the mainstream. It also might not be in line with the original spirit of the Pokémon series in some ways.
One possible plotline could be that while Ash is sick, his Pokémon have to take care of him. Pikachu might gather berries to make a special potion to help Ash get better. Another could be that Ash's friends are on a journey without him because he's too sick to travel, and they constantly worry about him and send back items to help his recovery.
One interesting plotline could be that Ash and Elesa decide to have a Pokémon - themed wedding. All the Pokémon they've befriended over the years could play different roles. For instance, Pikachu could be the ring - bearer. The invitations could be in the form of Poké Balls, and the venue could be decorated with different types of Pokémon elements. Another aspect could be that they discover a new Pokémon species during their wedding preparations. This new Pokémon has special powers that are related to love and unity, and it becomes a symbol of their relationship.
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.
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.
Well, in some Ash Ketchum high school fanfictions, there's a plotline where Ash is given the task of training a group of beginner Pokémon trainers in the school. He has to use his own experiences to teach them, which is not easy as they all have different personalities. Also, there could be a plot where Ash is framed for something he didn't do in the Pokémon - related incidents at school, and he has to prove his innocence while still focusing on his Pokémon training and studies.
One interesting aspect is his growth. In fanfiction, we can see how he becomes a more skilled trainer over time. For example, he might learn new battle strategies.
One possible plotline could be that Ash, as a girl, has to face different challenges in the Pokémon world. For example, some trainers might underestimate her at first. She could also form new friendships or rivalries. Maybe she'd have a different approach to training Pokémon, focusing more on strategy and less on brute strength like she sometimes did as a boy. Another aspect could be how her Pokémon react to her new form. Some might be more protective, while others could be confused at first but then adapt.