One possible storyline could be Ash and his Greninja. They could go on a special mission to save a group of Pokémon from a natural disaster. Along the way, Greninja and Ash have to rely on each other's skills and instincts, strengthening their bond. Another could be Ash and his male Snorlax. Snorlax gets sick, and Ash has to find a rare herb to cure it, going through many challenges in the process.
One possible storyline could be that Ash and Erika are on a journey to find a rare Pokémon that is said to have special powers. Along the way, they face various challenges like rough terrains and other trainers. Another could be that they are traveling to a Pokémon tournament together and during their travels, they help each other improve their Pokémon's skills.
Charizard could be a bit more indifferent. It's a proud and strong Pokémon. It might just snort when it sees Ash in a dress, as if to say 'This is a strange human thing, but it doesn't affect our relationship'. However, if other Pokémon made fun of Ash because of the dress, Charizard might get protective.
One possible plot twist could be that Ash wakes up one day and finds he has turned into a Pokémon himself. This would completely change his perspective and how he interacts with the Pokémon world.
One possible role is that he could be a spy. Since he was turned into a Pokémon, Team Rocket might not suspect his true identity and he could gather information about their evil plans. For example, he could overhear conversations between Team Rocket members that he couldn't have accessed as a human.
If Ash controls minds in the Pokémon fanfiction, it could lead to him being ostracized by other trainers. In the Pokémon world, such unethical behavior would not be tolerated. Other trainers might see him as a threat and try to stop him. This could create a lot of conflict in the story, with Ash having to either defend his actions or try to reverse the mind - control ability he has acquired.
One possible storyline could be Ash's journey after becoming the Alola champion. He might travel to other regions to share his Alola - inspired battle strategies. Another could be him mentoring new trainers in Alola, passing on his knowledge and experiences. And perhaps he could face challenges from trainers who want to take his champion title.
In this fanfiction, a great storyline could be that May's crush on Ash actually helps her grow as a person. She becomes more confident in herself and her Pokémon training because she wants to impress Ash. There could be a moment where Ash is in danger during a battle, and May's feelings for him give her the extra motivation to step in and save him, which then leads to a deeper connection between them.
Another storyline could involve a female Lugia being in trouble because some evil Pokémon trainers are trying to capture it for its power. Ash hears about this and teams up with his friends to protect the Lugia. Along the way, they face many challenges and Ash has to use his battle skills and knowledge of Pokémon to outwit the villains.
One possible storyline could be that Ash is a knight in training and Serena is a noble lady. Ash has to prove his worth in a jousting tournament to win Serena's hand in marriage. Another could be that they are both on a quest to find a magical Pokemon that is said to have the power to save the medieval kingdom from an evil sorcerer. And a third could be that Serena is a healer and Ash is a warrior, and they team up to fight off a horde of dark - type Pokemon that are attacking the villages.
Perhaps Manaphy gets lost, and Ash and May have to search through different regions to find it. During their search, they encounter new Pokemon species and make new friends who help them in their quest. This journey could also strengthen the bond between Ash, May and Manaphy.