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.
One interesting idea for the fanfiction is that May betrays Ash to protect her family. An evil group threatens her family and the only way to save them is to turn against Ash. This would create a complex situation where May is torn between her loyalty to Ash and her love for her family. Ash, on the other hand, would be completely blindsided by her sudden change in behavior.
It could be a rival trainer. Rival trainers are often very competitive in the Pokémon world. Maybe this rival is jealous of Ash and May's progress and decides to betray them to slow them down. They might spread false rumors about Ash and May, causing other trainers to be distrustful of them.
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 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.
Well, it could be that Ash Aura, who was perhaps trusted by the group in the fanfiction world, reveals some dark secrets or joins the enemy side, thus betraying those who once relied on him. Maybe he was lured by power or some hidden motives.
One possible plotline could be that Ash discovers May has been secretly working with a rival trainer in order to gain an advantage in a big tournament. This would make Ash feel betrayed as he thought they were friends and training partners. Another could be that May betrays Ash for a rare Pokémon that Ash was also after, choosing the Pokémon over their friendship.
In such a fanfiction, they might be betrayed by a so - called friend. For example, a fellow trainer could steal their Pokémon in order to gain an advantage in a big tournament. This would be a huge shock to Ash and May, and they would have to figure out how to get their Pokémon back and also deal with the sense of betrayal.
One possible plot could be that his mom reveals she has been working with an evil organization all along. She hands Ash over to them for some dark purpose, like using his unique Pokémon connection for their evil experiments. Ash would feel extremely hurt and betrayed, having to then find a way to escape and also come to terms with his mother's true nature.