Perhaps a new and powerful trainer arrives in the story. This trainer manages to deceive Ash's harem members, making them think that Ash is a hindrance to their growth as trainers. As a result, they side with the new trainer and betray Ash, leaving him feeling confused and alone.
Another plot could involve a magical or mysterious force that corrupts the minds of Ash's harem. This force makes them see Ash as an enemy instead of a friend. It could be related to some ancient Pokemon - related curse or a new and dangerous Pokemon's special ability.
One common plot is that Ash's friends or some of the female characters in his harem turn against him due to a misunderstanding. Maybe they think he has some ulterior motives or has betrayed their trust in some way. Another plot could be a new character coming in and manipulating the situation to make it seem like Ash is the bad guy.
One possible analysis could be that in such a fanfiction, the idea of Ash being betrayed might be a way to create drama. It could involve his friends or companions in the harem turning against him for various reasons like jealousy or misinformation.
One common theme is trust and its breakdown. Since Ash is betrayed, it focuses on how trust is lost among the characters in the harem.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). On FanFiction.net, use the search bar and type in 'Pokemon Ash Betrayed Harem' to see if there are any relevant stories. AO3 also has a large collection of fanfictions, and their tagging system can help you locate the specific type of story you're looking for.
One interesting plot could be Ash forming a unique bond with each Pokémon in his harem. For example, he might have special training sessions with different Pokémon that are tailored to their individual strengths and weaknesses.
One possible plot could be that Ash discovers Anabel has betrayed him by siding with a rival trainer in a crucial tournament. This could lead to a rift in their friendship and Ash having to find a way to overcome this setback and prove himself without her support.
One possible plot could be that Ash is falsely accused of a serious crime within the Pokémon world, and Greninja, due to some misinformation or outside influence, turns against him. Maybe a powerful villain manipulates Greninja's mind, making it believe that Ash is a threat to the Pokémon world. This could lead to intense battles between them, with Ash being heartbroken and trying to prove his innocence while also trying to save Greninja from the villain's control.
Perhaps Ash is wrongly accused of something in Skyla's hometown. Skyla believes the false accusations and turns her back on Ash, which makes Ash feel extremely betrayed. He then has to prove his innocence not only to Skyla but also to the whole town.
It could be a jealous rival trainer. This trainer might have always been envious of Ash's popularity and the loyalty of his harem. So, they plot and scheme to turn the harem against Ash by spreading lies and creating discord among them.
It's possible. However, when searching for it, make sure to read the summaries and reviews carefully. Some fanfictions might seem innocent at first but could contain some mild themes that might not be suitable for very young readers. Check for tags like 'All Ages' or 'General Audience' on the fanfiction platforms.