One possibility could be that in the fanfic, Diantha betrays Ash for power. Maybe there's a powerful organization that offers her something she can't resist, like control over all Pokémon battles in a certain region. She then uses Ash in some way to further her own goals, perhaps leading him into a dangerous situation that he wasn't aware of. This could lead to Ash feeling extremely hurt and betrayed, and it might cause him to question his own judgment in trusting people.
In the fanfic, it could be that Ash betrays Diantha. Suppose Ash discovers some dark secret about Diantha that he thinks is harmful to the Pokémon world. He might then choose to side against her, which she could perceive as a betrayal. For example, if Diantha was secretly involved in some unethical Pokémon experimentation in order to create stronger Pokémon for battles. Ash, being the defender of Pokémon rights, would not stand for it and take actions that Diantha views as betrayal.
Power struggle can also be a theme. If Diantha betrays Ash for power, it shows how power can corrupt and change a person's values. It could also lead to a battle of wills between them, as Ash might try to oppose Diantha's new -found power - hungry ways. This theme often explores the darker side of human (or in this case, trainer) nature when faced with the allure of power.
In a fanfic about Ash and Iris betrayal, perhaps Iris could betray Ash over a powerful Pokémon they both encounter. Maybe she feels that Ash is not strong enough to handle it and decides to take it for herself, thinking it will help her on her journey to become a top Pokémon trainer.
There could be a dangerous situation. Like they are in a place full of powerful, wild Pokémon, and Ash manages to protect Diantha. This act of heroism makes Diantha see Ash in a new light, and she can't help but kiss him. Or perhaps they are lost in an unknown region, and during their time trying to find their way back, they grow closer and closer until they kiss.
Well, they could be betrayed by a new acquaintance in a strange town they visit during their travels. This new person could pretend to be friendly, but then steal their Pokemon or important items.
One possible scenario is that Greninja is tricked into believing that Ash is endangering its kind. So it decides to turn on Ash. Haruka, on the other hand, having known Ash for a long time, might try to convince Greninja of Ash's true nature. However, Greninja's loyalty is so shaken that it doesn't listen easily. In the end, Ash has to prove his worth and friendship to Greninja all over again.
One common theme is adventure. They often go on Pokemon - related adventures together. Another theme is friendship development. They start as acquaintances or fans of each other and gradually become close friends or more. And there's also the theme of mutual respect, as they both recognize each other's skills and achievements in the Pokemon world.
In this fanfiction, the betrayal might start with Mew stealing a powerful and unique Pokemon from Ash. Mew, being a rare and powerful Pokemon itself, could have a complex motive. Maybe it wants to prove its superiority by taking something precious from Ash. Ash would be heartbroken and angry. He would then embark on a journey to get his Pokemon back, facing many challenges along the way and ultimately having a confrontation with Mew where he tries to understand why it betrayed him and if there's any way to fix their relationship.
One possible storyline could be that Ash discovers Lillie has been secretly working with a rival trainer group. This betrayal could stem from Lillie's desire to protect something or someone she cares about in a way that Ash doesn't initially understand. Maybe she believes the rival group can offer a solution to a problem that Ash's methods can't. For example, if there's a threat to the Alolan Pokémon, she might think the rival group has better resources.
The 'pokemon ash x diantha fanfic' can be a great exploration of their potential relationship. It could start from their first encounter, which might be full of surprises. Then, as the story progresses, we might see how they grow closer. Their different backgrounds, with Ash being a young trainer from Pallet Town and Diantha being a famous champion, create a lot of room for interesting storylines. For example, Ash could learn a lot from Diantha's more refined training methods, and Diantha could be inspired by Ash's unwavering enthusiasm.
Well, they could have an epic battle side by side again. Maybe they face a powerful new enemy that only their combined skills can defeat. Ash could show how much he's grown since they last parted, and Greninja might have some new moves or abilities too.