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.
Often, there are plotlines where Ash teams up with unexpected allies. He might join forces with a group of trainers who have different skills and knowledge. Together, they face powerful opponents and through these experiences, Ash learns new techniques and strategies, thus getting stronger. It could also involve Ash mentoring a younger trainer and in the process, he discovers new ways to improve himself.
One plotline could be how the other characters in the story react to Sans being sick. For example, Papyrus might be overly worried and try all sorts of crazy remedies. Another could be Sans trying to hide his sickness because he doesn't want to be a burden.
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.
One possible plotline could be Ash's discovery of his own latent legendary powers. Maybe he starts to exhibit abilities that are unique to legendary Pokemon, like controlling the weather or having an extraordinary bond with all types of Pokemon.
One common plotline is that Ash is chosen by a legendary pokemon for a special mission. For example, he might be chosen by Lugia to stop an evil organization from misusing the power of the sea. Another is that Ash stumbles upon a legendary pokemon in a hidden or dangerous place, like in a long - lost ancient ruin, and forms a bond with it.
Another plotline could be Ash getting involved in some sort of virtual Pokemon world within the fanfiction that has unique rules and challenges. He could discover new Pokemon species that are exclusive to this gamer - themed world. This would add a whole new dimension to his adventures and his relationship with his existing Pokemon team as they all adapt to this new environment.
One interesting plotline could be Ash and Alexa going on a special journey to discover a new type of Pokemon. They might face challenges like difficult terrains and other trainers trying to stop them.
In some fanfictions, there could be a plot where Ash has to protect Lillie from a group of evil Pokemon trainers. This would show his loyalty and courage, and also lead to some exciting battles and adventures. And their relationship could grow deeper through this process.
One interesting plotline could be Ash coming back with a new and powerful Pokemon team, ready to take on new challenges in the Pokemon world. Maybe he has a super rare Pokemon that no one has seen before.
One interesting plotline could be Ash facing new and extremely powerful Pokémon that he has to strategize in unique ways to defeat. Maybe he discovers a hidden Pokémon species that has been lost for centuries and has to figure out how to befriend it. Another could be Ash getting involved in a Pokémon tournament with a much higher level of competition than he's used to, which forces him to rapidly improve his skills.