Perhaps Ashley betrays Ash by revealing his battle strategies to other trainers. In the Pokemon world, strategies are crucial, and this could lead to Ash losing battles. As a result, Ash might feel extremely disappointed and have to come up with new strategies while also dealing with the betrayal.
It could be that Ashley was working undercover for an evil organization all along. She pretends to be Ash's friend but then at a critical moment, she hands over important information about Ash and his Pokemon to the organization. This would put Ash and his Pokemon in great danger, and he would have to fight not only to protect himself but also to expose Ashley's true motives.
Perhaps Leaf betrays Ash because she becomes jealous of his friendship with Pikachu. She might feel like Pikachu gets all of Ash's attention and decides to side with a group that wants to separate them. This could lead to Ash having to fight not only to save Pikachu but also to win Leaf back.
In 'Goku betrayed fanfiction', a common plot might be the corruption of his power. Let's say Goku discovers a new form of power that is seen as too dangerous by others. His friends and even some of the Z - fighters fear that he'll lose control and turn on them. So they decide to betray him first to protect the Earth. Another aspect could be a political betrayal. If there were some sort of intergalactic politics involved, Goku might be sacrificed or betrayed by his own side to make a deal with a more powerful alien race. And then there's the idea of a time - travel - related betrayal. Maybe someone from the future comes back and convinces everyone that Goku will cause a great disaster in the future, causing them to betray him in the present.
Maybe there is an evil Pokemon trainer who has influenced Ashley to betray Ash. In this case, that trainer would be the main antagonist. He or she might have promised Ashley something in return for her betrayal, like rare Pokemon or power.
One possible storyline could be that Percy is tricked into betraying Tartarus by some dark force. Maybe they use his friends as hostages or manipulate his mind. Another could be that Percy believes he's doing the right thing in betraying Tartarus, but later realizes it was a huge mistake and has to face the consequences. For example, he might face the wrath of Tartarus' minions or lose the trust of his allies.
Perhaps in the fanfic, Sakura betrays her comrades due to a misunderstanding. She might be framed by an enemy as if she has leaked important information. This leads to her being ostracized by her friends, and she has to fight to prove her innocence while dealing with the emotional turmoil of the false accusation.
One possible storyline could be that Natsu is wrongly accused of a crime within the Fairy Tail guild. Maybe some false evidence is planted against him, leading the guild members to turn their backs on him. Another could be that a powerful enemy manipulates the guild into believing Natsu is a threat, so they betray him. It might also be that there's a misunderstanding about Natsu's powers or intentions, causing the betrayal.
Maybe in the fanfiction, Gladion is from a noble family in the Pokemon world. His family has certain expectations for his wedding. He, however, chooses to marry someone who is not from a noble background. There could be a lot of drama with his family trying to stop the wedding at first. But in the end, they see how much he loves his partner and give their blessing.
In a more complex story, there could be a curse placed on Pikachu by an ancient Pokemon spirit. This curse gradually weakens Pikachu over time. As it gets sicker, Ash tries everything he can to find a cure. But in the end, Pikachu's body gives out. Ash is left to deal with the grief, and he embarks on a journey to find a way to break the curse and bring Pikachu back.
Another idea could be that Dawn gets a haircut that has a special meaning in a certain Pokemon culture. For example, she visits a new region where a particular haircut is a sign of respect for a powerful Pokemon deity. She decides to get that haircut to show her respect and in doing so, she gains the favor of the local Pokemon trainers and their Pokemon. They start to share their training secrets with her because they see her as someone who respects their traditions.
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.