One common theme is the power dynamics. Hypno often has a certain power that affects Ash and the characters in the harem. Another theme could be the relationships that develop within the harem. They might start with conflict due to jealousy or different goals but gradually form a more harmonious group.
Definitely Pikachu. It's Ash's closest companion, so it could be part of the harem in a very unique way. Then there could be female Pokémon trainers like Misty and May. They have a history with Ash and could be part of this harem concept.
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.
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.
In some 'harry potter hypno harem fanfiction', there could be a plot where a magical object has the power of hypno. It gets into the wrong hands and starts to influence the students at Hogwarts. The main character then has to navigate through the mess of relationships that start to form within the harem - like dealing with jealousy among the hypnotized characters, all while trying to figure out how to break the spell on the object.
Ron Weasley could be there too. He might be a bit more clueless at first about the hypno harem situation but would still be dragged into it. And of course, there could be some original characters created just for these fanfictions. They could be new students or mysterious wizards who are either causing the hypno or are part of the harem that forms.
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.
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.
A common theme in such fanfictions could be Ash's adventures in Alola while building relationships with different characters. Maybe he forms special bonds with his Pokémon and the people around him in a more romantic or friendship - deepening way.
Well, they might feel a range of emotions. Some could be angry, feeling that they were wronged after putting their trust in Ash. Others might be heartbroken, especially if they had deeper feelings for him. For example, one of the harem members who was really close to Ash might go into a state of denial at first, not wanting to believe that he could do such a thing.