Maybe there's a dark Pokémon or a Pokémon under the control of an evil force. This Pokémon could be attracted to the strange energy of Ash's transformation and tries to keep him in his new form for its own mysterious purposes. It could constantly create obstacles for Ash as he tries to find a way to reverse the transformation.
There could be a group of Pokémon poachers who mistake Ash, in his new female form, for an easy target. They might try to capture his Pokémon while he's dealing with the changes in his body and abilities. Or perhaps there's a jealous trainer in Erika's gym who doesn't like the attention Ash is getting, even in his new form, and tries to sabotage his attempts to turn back.
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.
A group of rival trainers could be the antagonists. These trainers are jealous of Ash's skills and Erika's knowledge. They constantly try to sabotage their journey, for example, by setting traps or spreading false rumors about dangerous Pokémon in the areas they are about to travel through.
One possible plot could be that a spell or a magical artifact causes Ash to turn into a girl. Ericka might be involved in the discovery of this transformation. Ash then has to deal with the new physical form while still trying to pursue his Pokemon journey. His Pokemon might initially be confused but still recognize him. And he could face different challenges and reactions from his friends and rivals as a girl.
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.
Well, he might start to see the world from a Pokemon's perspective. For example, he could understand their communication better and form closer bonds with them. He may also face new challenges like having to adapt to living in a Pokeball or dealing with the natural instincts of his new Pokemon form.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
It could be a jealous trainer who doesn't like the attention that girl - Ash is getting. This trainer might try to undermine her at every turn during Pokemon battles.
There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.
His best friend Pikachu might be a bit confused at first but would quickly adapt as it still senses Ash's essence. Brock would probably be very surprised and might offer some comical comments or advice on how to handle the new situation. Misty could be a mix of shocked and intrigued. She might be a bit jealous of Ash's new female appearance at first, but then would support him as they are friends.
The Marines could still be antagonists. They might view Luffy's transformation as a sign of weakness or an opportunity to capture her more easily. They could underestimate her because she is now a girl, but of course, they would be wrong.