In the fanfiction, Ash could be so sick that he temporarily loses his ability to communicate with his Pokémon. This causes his Pokémon to become worried and confused. However, they don't give up. They try different ways to get through to Ash, like using their unique moves in a non - combat way. Meanwhile, Team Rocket might take advantage of Ash's situation at first, but then they have a change of heart and end up helping Ash's friends to find a cure.
Maybe Ash's sickness is caused by a new and mysterious Pokémon. As he's sick, he starts having strange dreams about this Pokémon. His friends then start to investigate based on the details from his dreams. They discover that this Pokémon has some sort of special power that is making Ash ill. In order to cure Ash, they have to find and befriend this Pokémon. They go through a series of adventures, facing other Pokémon and trainers, all while Ash is lying sick in a Pokémon center.
In the fanfic, it could be that Arceus has its own reasons for allowing Ash to catch it. Maybe it sees Ash as the key to restoring balance in a world that is slowly being corrupted by human greed and over - exploitation of Pokemon resources. So, as Ash catches Arceus, he is thrust into a mission to cleanse the world. He might have to travel to different regions and use Arceus's power to heal the damaged environments and reconcile the differences between humans and Pokemon.
Ash could become more understanding of different training methods through Daisy. Maybe he learns from her how to better communicate with his Pokemon on an emotional level.
It could be that Ash has a special mission given to him by a Pokémon deity or an important figure in the Pokémon world. The mission is to bring a specific set of Pokémon to Alola for some ecological or cultural reason. Maybe there are certain Pokémon that can help balance the ecosystem in Alola, like Pokémon that can control overgrown plants or deal with an overpopulation of certain species.
One possible plot development could be that his friends, like Sakura and Sasuke, come to take care of him. Sakura uses her medical skills to try and bring down his fever. Another could be Naruto having fever-induced dreams about his past, like his lonely days as a child. Maybe he even hallucinates about his parents. It could also lead to a situation where the village is in danger, but Naruto is too sick to help at first, creating a sense of tension and urgency among the other characters.
Ash could develop his ability to understand different Pokemon cultures better. Cynthia, being from a different region, has a lot of knowledge about various Pokemon traditions. Ash could learn from her and in turn, teach her about his own experiences. This exchange could make Ash more worldly and Cynthia more in touch with the broader Pokemon community. In terms of their relationship, they could start to rely on each other more, which would be a big step for both of them character - wise.
A possible plot development could be that Gible evolves during their time in Kalos. This would give Ash a more powerful Pokemon to compete in Kalos gym battles. Another development could be that Gible discovers a new type of move or ability while in Kalos, which gives Ash an edge in battles.
One possible plot development could be that his friends rally around him. For example, Sakura might use her medical skills to try and figure out what's wrong. Another could be that Naruto's illness is somehow related to a new enemy or an old foe's hidden jutsu. It might also lead to Naruto having to rely on others more than he's used to, which could be a big character - growth moment for him.
There's a fanfic titled 'When Ash Fell Ill'. This one focuses more on Ash's internal struggle while being sick. He feels useless and is frustrated that he can't go out and catch more Pokemon or train. But his Pokemon stay by his side, and through their bond, he slowly starts to recover. It's a heart - warming story about the relationship between Ash and his Pokemon.
Ash might develop a deeper understanding of different training styles. Bianca has her own unique way of training Pokemon in the Unova region, and through their interactions, Ash could incorporate some of her ideas into his own training. Bianca, on the other hand, could become more adventurous. Ash's tales of his travels could inspire her to explore more areas in Unova and take on greater challenges. Their relationship could also lead to both of them becoming more empathetic towards others, as they support each other through various difficulties.
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.