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.
It could be that there is a young, impressionable character who looked up to Ash Aura. After the betrayal, this character is confused and starts to question his own beliefs. There might also be a powerful mage in the story who has the ability to see the truth behind Ash Aura's actions and is trying to prevent the chaos that the betrayal has caused.
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.
One possible plot could be that Ash discovers his hidden ability to be an Aura Guardian gradually. Maybe he starts having strange dreams about it at first. Then, in a battle, he unconsciously taps into his aura power to save his Pokémon. This new power would lead him on a journey to understand more about the Aura Guardians, their history and his place among them.
Well, it could be about a character named Ash who discovers his aura power in a time of great danger. Maybe his friends are in trouble and this newfound power is the key to saving them.
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.
In a 'ash and pikachu snivy betrayed' fanfiction, the betrayal could stem from a misunderstanding. Maybe Snivy overhears Ash making a plan that it misinterprets as something harmful to the Pokémon world. As a result, Snivy feels it has no choice but to betray Ash and Pikachu to protect what it believes in. It could then side with a group of anti - human Pokémon who are also misinformed about Ash's intentions.