Perhaps in the fanfiction, Ash was betrayed by a powerful mentor figure. This mentor, who was supposed to guide him, saw Ash as a threat to their own power or plans. So they leaked Ash's secrets to his enemies, which led to his downfall. It could also involve a love interest. Say, a girl Ash trusted and liked turned out to be working for the villains all along and used his feelings for her to betray him.
In this 'ash the chosen one betrayed fanfiction', one possibility is that the Pokémon world itself betrays Ash. There could be some ancient prophecy or dark force at work. The very Pokémon that Ash has befriended and trained could be turned against him. This could be due to some kind of mind - control or a curse. The people in the Pokémon world who once supported Ash might start to fear him because of this new situation and turn their backs on him, leaving him all alone against the threats.
His rival could be the one to betray him. Rivals are often jealous and competitive. If Ash's rival saw an opportunity to gain an advantage by betraying him, they might do it. For instance, they could lie about Ash to the Pokémon League, getting him disqualified from important battles.
Well, Ash could lose his trust in people. If he's betrayed, he might become more guarded and less likely to form new friendships. This could change his entire approach to his Pokémon journey. He might start to rely more on his Pokémon for emotional support rather than other humans.
One example could be a story where Ash is betrayed by his so - called friends in the Pokémon world who are jealous of his status as the chosen one. They might spread false rumors about him, causing him to lose the trust of the people around him.
One idea could be that Ash is betrayed by his so - called friends in the Pokémon world. They might be jealous of his status as the Chosen One and team up against him. For example, Gary could spread false rumors about Ash, causing other trainers to doubt him.
The development could involve a complex plot. First, establish Ash's journey as the chosen one. He has a mission, and he's making progress. But then, an unexpected entity betrays him. This could be a spirit or a powerful trainer. They use Ash's trust against him. For example, they lead him into a dangerous situation under the pretense of helping him. As a result, Ash loses his confidence and has to start from scratch to regain his status and trust. The story can then focus on his redemption and how he overcomes the betrayal.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.
Perhaps May betrays Ash because she's influenced by a new group of trainers. They might convince her that Ash's way of training Pokémon is wrong or that he's holding her back from achieving greater things. So, she sides with them against Ash. This would show how easily friendships can be tested in the competitive world of Pokémon training.
Perhaps Pikachu discovers a 'dark truth' about Ash. For example, Pikachu might find out that Ash has unknowingly been involved in something that goes against Pokémon ethics, like being associated with a group that mistreats Pokémon. This discovery could make Pikachu feel betrayed by Ash first, and then Pikachu's actions would make Ash feel the same way in return.
In this fanfiction, it might be that Ash has been friends with Lemon, but then Ash sides with an enemy of Lemon's for some false gain. He might think that the enemy can offer him something better, like power or wealth, and in doing so, he stabs Lemon in the back by giving away Lemon's plans or weaknesses to the enemy.
In an Ash betrayed champion fanfiction, perhaps the champion has been secretly working with a villainous group. The champion lures Ash into a trap under the pretense of a friendly battle. Ash, being trusting, falls for it. He then has to find a way to escape and get revenge. He might meet some new allies along the way who help him see the champion's real motives.