It could be that the main antagonist is a Hypno itself that has gone rogue. It has developed this 'L to obey' power on its own due to some strange mutation or exposure to a powerful energy source. This Hypno is using its power to make other Pokémon serve it, and Ash is the only one who can stop it. Ash would need to use his wits and the unique bond he has with his Pokémon to defeat this powerful Hypno.
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.
One possible plot could be that Ash gets caught in a strange situation where a Hypno uses its powers on Pikachu to make it disobey Ash. Ash then has to find a way to break the hypnosis and regain Pikachu's loyalty. Maybe he teams up with some other Pokémon or trainers he meets along the way.
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.
Well, this type of fanfiction concept often delves into the realm of the psychological. The idea of mind control over Ash Ketchum can be seen as a way to play with power dynamics. If a villain is the one controlling him, it gives the villain an upper hand in a different way compared to just physical strength. And it forces the fanfiction writer to think about how the Pokémon world's rules and magic could be bent or used in relation to hypnosis. Also, it can open up opportunities for exploring different moral and ethical questions within the story, like whether it's right to use mind control, and how far his friends should go to free him.
The plot could involve a new and powerful villain who discovers a way to hypnotize Ash. This villain might want to use Ash's skills and knowledge of Pokémon to achieve their own evil goals, like stealing rare Pokémon or taking over a Pokémon gym. Ash, under the mind control, starts to obey the villain's commands without question, which alarms his friends. His Pokémon might sense something is wrong and try to resist following Ash's now - controlled commands.
Maybe a new, mysterious villain who has a grudge against Ash or wants to use his skills for their own evil purposes. This villain might be from a far - off region and has special powers that allow them to hypnotize people. They see Ash as a useful tool to carry out their plans in the Pokémon world.
One possible plot could be that a villain uses hypnosis on Ash Ketchum to make him do their bidding in Pokémon battles. So instead of using his own strategies, he's forced to follow the commands of the hypnotist. This would create a lot of chaos and challenges for his friends as they try to figure out what's wrong with Ash and how to break the hypnosis.
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.
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.
The plot could be that an unknown villain hypnotizes Ash. The villain wants to use Ash's skills and reputation to steal Pokémon. Ash, under hypnosis, starts to lead other trainers into traps. His friends are confused at first but then realize he's not himself. They search for a way to reverse the hypnosis, perhaps by finding a rare Pokémon or a special item.
Well, if Ash was hypnotized to obey commands in fanfiction, it might change the whole dynamic of his adventures. For example, Team Rocket could get hold of the hypnosis method and try to make Ash work for them. But Ash being Ash, even under hypnosis, his love for Pokémon might break through the commands and he could end up turning the situation around on Team Rocket.