It could be an Alolan Pokémon that has some sort of mind - controlling ability. In Kalos, Haruka unknowingly gets exposed to this Pokémon's power. By the time they get to Alola, the Pokémon fully takes over Haruka's mind at times and makes her do things that betray Ash, like leading him into dangerous traps or stealing his Pokémon.
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.
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.
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 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.
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.
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.
It could be a new character, someone who has a long - standing grudge against Ash's group. This character might manipulate Ash into betraying his friends by preying on his insecurities or desires.
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.
It could be Gary. Gary has always been a rival to Ash. Maybe in this fanfiction, Gary gets jealous of Ash's progress and decides to betray him by making false accusations to the Pokemon League about Ash using illegal training methods.
A fellow trainer in Alola could betray Ash. This trainer might be envious of Ash's success as a champion. He could secretly work with Team Skull or some other antagonists to undermine Ash. For instance, he might give false information to Ash about a battle location, leading Ash into a trap.