He could interact with Serena again. Since they have a history in Kalos, they might have new adventures together. Also, Clemont and Bonnie are likely characters he would meet up with. They could continue their friendship and maybe work on some new inventions together with Clemont, or have fun times with Bonnie and her love for Pokémon.
Ash might also encounter some new trainers in Kalos that are not part of the main story. These could be trainers who are on their own unique journeys, and Ash could learn different training styles from them. He could also interact with the Elite Four members of Kalos, getting advice from them on how to become an even better trainer.
One possibility is that Ash decides to stay in Kalos to study Pokémon evolution in depth. Kalos has some Pokémon with very interesting evolution methods. He could team up with local Pokémon researchers and have adventures while learning more about how different Pokémon evolve under various conditions. This would not only expand his knowledge but also strengthen his bond with his own Pokémon.
He could meet Serena, who might be his travel companion or a friendly rival. Also, Clemont and Bonnie are likely characters he'd encounter. They could help Ash with his journey in Kalos, like Clemont with his inventions and Bonnie with her cheerful nature.
Obviously, Ash himself. And his Kalos friends like Serena would probably be there. Clemont and Bonnie might also make an appearance as they were important parts of Ash's Kalos journey.
Serena could be one of the main characters in the harem. She has a close relationship with Ash already in the original series. Also, Miette might be included as she showed interest in Ash. And Shauna could also be part of it as she was always friendly towards Ash during his Kalos journey.
Well, one possibility is that Ash continues his Pokemon training in Kalos, facing new and stronger opponents. Maybe he teams up with old friends from Kalos to take on a new threat to the region.
In Kalos, one potential rival for Ash could be a local trainer who specializes in Fairy - type Pokémon. Fairy - type Pokémon can be quite tricky in battles, and this trainer has mastered the use of their unique abilities. For Misty, a rival could be a girl who is an expert in water - type Pokémon battles, just like Misty herself. This girl might have some rare water - type Pokémon from Kalos that Misty has never seen before.
Definitely Misty could be one. She has a long - standing relationship with Ash and might be part of the harem as she still has feelings for him. Another could be Serena, who has shown strong romantic interest in Ash in the past. Dawn might also be a character, as she and Ash have a great friendship that could potentially turn into more in a harem - like scenario.
Lillie is a likely character. She could befriend Ash and they might go on Pokémon adventures together. She has her own story and growth in the Alola region, and interacting with Ash could add more depth to both of their characters. Also, Ash might meet the Trial Captains like Mallow or Sophocles. They could challenge Ash in different ways and teach him about the island challenges.
One possible fanfic could be about Ash's journey to become the Kalos champion. It might start with his arrival in Kalos, full of excitement and determination. He meets new Pokémon and friends along the way. His battles are intense, with him using unique strategies to defeat his opponents and gradually climb the ranks to the championship.
Well, a fanfic about Ash, Misty and Charizard in Kalos could be really exciting. Maybe it could start with Ash arriving in Kalos with Charizard, and then Misty comes to visit. They could explore the unique Pokémon in Kalos together, and Charizard might have some interesting battles against the local strong Pokémon. It would be a great story to show their friendship and adventures in a new region.