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.
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.
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.
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.
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.
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.
He could meet Serena, who is from Kalos. She might become an ally and help Ash understand the Kalos region better.
Ash might meet some lone Pokémon trainers. These trainers could be on their own quests, similar to Ash's. They might have unique Pokémon or training methods that Ash can learn from.
He might meet Serena again. Since he has been to Alola and now travels to Kalos, their reunion could be an interesting plot point.
He might meet some local Sinnoh trainers who are experts in training specific types of Pokémon, like a trainer who specializes in ice - type Pokémon in the colder regions of Sinnoh.
There might be a young and inexperienced trainer who is lost. Ash, being the more experienced one, decides to help the trainer find their way. This new character could look up to Ash and in the process, Ash learns more about himself as a role model. It would be an interesting dynamic as Ash is usually the one being guided by his friends.