In a fanfiction, Ash could also develop a different side of his personality. When he is with Daisy, he might show more of his sensitive and caring side that he doesn't always display during his normal Pokemon journey. Daisy, on the other hand, could develop her leadership skills as she and Ash face various challenges together, and she starts to take charge in certain situations.
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.
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.
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.
Ash could become more patient. Since Togepi is a relatively weak and cute Pokémon at first, Ash has to be patient in training it. This would be a change from his sometimes impulsive nature in battles.
Ash might develop a deeper sense of responsibility. In a fanfiction, if he is in charge of protecting Manaphy, he would have to think more about the consequences of his actions. He could also learn to be more diplomatic, especially when dealing with May's different approach to Pokémon. May could grow in her ability to adapt. For example, if they end up in a situation where there are no contests but only battles, she could learn from Ash how to handle it. Manaphy could develop its psychic abilities further. Maybe it discovers new powers within itself while being with Ash and May, and they help it to control and use these powers for good.
As a Mew fan, Ash might start to study different aspects of Pokémon lore more deeply. This could make him wiser in the ways of Pokémon. He could also develop a more empathetic side towards Mew - like Pokémon, seeing them as not just creatures to battle with but also as beings with their own unique qualities and stories.
For Pikachu, it could develop from being a bit reckless to more cautious. Maybe after getting into trouble due to its impulsive use of Thunderbolt in a wrong situation. And for Buneary, it could become more outgoing. For example, if it was initially shy around other Pokemon but as it spends more time with Pikachu, it starts to open up and make more friends.
For May, she could become more empathetic. Taking care of Manaphy would make her more in tune with the feelings of Pokemon, especially those that are rare and vulnerable like Manaphy. She might also become more confident in her abilities as a trainer, as protecting Manaphy from potential threats would require her to use her skills and strategies more effectively.
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.
Ash could become more mature. In a love - centered fanfiction with Cynthia, he might start to think more about the future and less about just the next Pokémon battle. His relationship with Cynthia could make him more strategic as he learns from her.
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.