Well, one possible plot could be that in the Kalos region, Ash makes a new powerful rival. Haruka, being in Kalos for some reason, gets influenced by this rival's charm and power. In Alola, Ash discovers that Haruka has been secretly helping this rival against him, thus feeling betrayed. Maybe the rival promised Haruka something she couldn't resist, like a rare Pokémon or a chance to become a top - tier trainer in a different way.
In this fanfiction, it could be that while in Kalos, Haruka falls in love with a trainer who has a grudge against Ash. When they all end up in Alola, this trainer manipulates Haruka into betraying Ash. Haruka, blinded by her feelings, starts to undermine Ash's efforts in Alola, like preventing him from participating in important battles or stealing his Pokémon training strategies and giving them to her new love interest's team.
The antagonist could be a powerful and devious Team Rocket member who has infiltrated the Kalos and Alola regions. This member could manipulate Haruka, perhaps by blackmailing her with something from her past in Kalos. In Alola, the Team Rocket member uses Haruka to get to Ash, making her betray Ash without really wanting to.
He might meet Serena again. Since he has been to Alola and now travels to Kalos, their reunion could be an interesting plot point.
In fanfiction, a story of Ash, Haruka (May in the English version), and Dawn could involve betrayal and hurt in many ways. For example, perhaps Ash makes a promise to both Haruka and Dawn about their Pokémon journey together, like he'll always be there to support them in battles and training. But then, he might break that promise by suddenly leaving them for a more 'glamorous' opportunity in another region without any prior notice. This would make them feel betrayed and hurt, as they relied on him not just as a friend but also as a partner in their Pokémon adventures.
In this fanfiction, Ash might encounter new Pokémon species in Kalos that he didn't see in Alola. He could also meet different trainers with unique battling styles.
One possible plot could be that Ash discovers Haruka and Hikari have been secretly working with a rival trainer. They've been sharing Ash's training secrets in an attempt to make the rival more successful. Ash feels betrayed because he thought they were his friends and trusted them with his strategies.
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.
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.
When writing about Ash, Haruka and Greninja's betrayal in fanfiction, you can start with a normal day in their journey. Then, something unexpected happens that leads to Greninja's change of heart. You need to build up the tension and drama. Use dialogue effectively to show the conflict between the characters. For instance, the conversations between Ash and Greninja when the betrayal occurs. And Haruka can be the mediator or the one who takes action to fix things. Also, think about the setting and how it can affect the story. A spooky forest or a mysterious cave might add to the atmosphere of betrayal.
It could be portrayed through their interactions. For instance, if Ash betrays them, their facial expressions when they see him next could show the hurt. Haruka might look away with a pained expression, and Dawn could have tears in her eyes. Their words could also be sharp and accusatory, like 'How could you do this to us, Ash?'
Well, it could be about Ash going back to Kalos to compete in some new Pokemon battles. Maybe he wants to challenge new trainers or participate in a special tournament there.