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.
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.
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.
Well, it could be about Ash exploring the new region of Kalos, meeting new Pokémon and trainers. Maybe he'll participate in the Kalos Pokémon League and face some tough opponents with his Pikachu and new Kalos Pokémon he catches along the way.
The plot could be that Ash enters a Kalos - only Pokémon competition with Charizard. They face strong opponents with unique Kalos Pokémon. Charizard might learn new moves by observing the local Pokémon. For example, it could see a Delphox use a powerful fire - type move and try to adapt it to its own style.
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.
He might meet Serena again. Since he has been to Alola and now travels to Kalos, their reunion could be an interesting plot point.
Well, it could be about Ash exploring the unique Pokémon in Alola. Maybe he discovers new forms of old Pokémon and has exciting battles with local trainers.