Another idea is that the main antagonist could be a Pokémon itself. A large and powerful Pokémon that sees Ash's growing strength as a challenge to its own dominance. It could constantly attack Ash and his friends, forcing Ash to not only rely on his Pokémon but also his own physical strength to defend and fight back.
One possibility could be a trainer who is jealous of Ash's new - found muscle - powered abilities. This trainer might see Ash as a threat to their own status in the Pokémon world. They could try to undermine Ash by using underhanded tactics during battles, like trying to tire him out before the fight even starts.
Perhaps a new organization that wants to use Ash's muscle growth for their own evil purposes. They might try to capture him and experiment on him to figure out how to replicate the muscle growth in others. This organization could be made up of scientists who have gone bad and believe that enhanced physical strength is the key to ruling the Pokémon world.
Well, it could be about Ash going through intense training regimes in different regions. Maybe he teams up with Brock who has some knowledge of bodybuilding - Brock could teach him about proper diet and exercise. And as Ash trains, he slowly builds up his muscles, which helps him in his Pokémon battles in a new and unexpected way, like being able to carry heavier Pokémon or move faster with added strength.
One thing that could inspire such a fanfiction is Ash's growth as a character. In the original series, he has grown in terms of his skills as a Pokemon trainer. Expanding on that, the idea of his physical growth, like muscle growth, could be a natural progression. Maybe he faces a new rival who is not only strong in terms of Pokemon battles but also physically intimidating. This could drive Ash to work on his own physical strength. Another inspiration could be if he enters a new tournament that has physical challenges as part of the preliminaries. To compete, he has to build muscle.
A fanfiction about Ash Ketchum's muscle growth could be really interesting. It might explore how Ash, who is usually depicted as a rather scrawny and young trainer in the Pokemon series, could start to build muscle. Maybe he decides to take on some physically demanding tasks in the Pokemon world, like helping to build new Pokemon habitats or training in a more intense way for upcoming battles. This could lead to his gradual muscle growth over time, and the fanfiction could detail his journey of self - improvement both physically and in terms of his confidence as a trainer.
Maybe a rogue Pokémon trainer who has a grudge against Ash. This trainer has dabbled in some dark and forbidden Pokémon training techniques that allow them to control minds. They target Ash because he represents everything they are not - kind, successful, and loved by many Pokémon.
The main antagonist could be a rogue Pokémon trainer who has a special Hypno that has been enhanced to have this 'L to obey' power. This trainer might want to use this power to take over Pokémon battles and tournaments, so Ash has to stop him.
It could be a rival trainer who has feelings for May and tries to drive a wedge between her and Ash. This rival might spread false rumors about Ash to make May doubt him, leading to Ash's heartbreak.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.
There could be a mad scientist - type character who created the situation where Ash turned into a Latias in the first place. But then, this character wants to keep Ash in that form to study him and use his new - found Pokemon abilities for their own gain. They would be the main obstacle for Ash in his journey to turn back into a human.
Ash himself could be the main antagonist as he is the one doing the betraying.
A new, mysterious organization that is interested in controlling the legendary Pokémon. They see Latias as a key to their plan and Ash as an obstacle to be removed.