Well, perhaps it could be about Ash getting cursed by Erica and then turning into a girl. And then he - or now she - has to deal with all the changes in her life, like how people around her react differently. Maybe she has to adapt to new social situations and new relationships all while trying to find a way to turn back.
Obviously, Ash and Erica would be the main characters. Ash is the one who undergoes the transformation, so he's central to the story. Erica is the one who causes the change, so she also plays a crucial role.
In fanfiction where Ash turns into a girl, it could lead to some interesting plot twists. Maybe his relationships with his Pokémon would change in unique ways. For example, some Pokémon might be more protective of him now that he's a girl. Also, his interactions with other trainers, like Misty and Brock, would be different. They might treat him with more chivalry or have different emotional responses to his new form.
Well, perhaps in the fanfiction, Ash is exposed to a mysterious Pokémon's special ability that causes the transformation. As a girl, Ash discovers new aspects of himself. He might find that some Pokémon are more attracted to his new form and are more willing to be caught or trained. He could also use this new identity to his advantage in battles, like using the element of surprise against opponents who are used to facing the male Ash.
I'm not sure which specific 'ash turns into a girl' story you are referring to. There could be various fictional or creative interpretations. Maybe it's a magical transformation where Ash, perhaps a character from a particular universe, is under a spell or has some special encounter that causes the change. Without more context, it's difficult to tell the exact story.
Maybe Team Rocket could be the main antagonist. They might see Ash's transformation as an opportunity to finally capture Pikachu. Since Ash is in a more vulnerable state as a girl, they could try to use different tactics to take advantage of the situation.
One possible theme could be identity. Ash, now as a girl, would have to re - evaluate who he/she is. Another theme might be relationships. His/her relationships with Gary and other characters would likely change in interesting ways.
It could be that there's a prophecy or a mysterious force at work in Erika's garden. Ash, being his curious self, stumbles upon it and it transforms him. The fanfiction might explore how his Pokémon react to his new form. Some might be confused, while others could be protective. Ash, as a girl, would have to face challenges like different expectations in Pokémon battles and dealing with rivals who see him as an easy target now.
In an 'Ash turns into Latios' fanfiction, perhaps Ash is turned into Latios as a way to better understand the Pokémon he has befriended. As Latios, he might be able to communicate with other Pokémon on a deeper level. For example, he could learn the secrets of legendary Pokémon that have always been a mystery. His friends, like Pikachu and Misty, would initially be shocked but then have to figure out how to help him reverse the transformation or support him in this new form.
I'm not sure of specific 'Gary turns Ash into a girl' fanfictions. You can try searching on fanfiction - specific websites like FanFiction.net or Archive of Our Own. Just use those keywords in their search bars and you might find some interesting works.