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.
He might meet new Pokémon trainers in Alola. Maybe some young kids who are just starting their own Pokémon journey.
There could be an old wise man or woman in the Alola region who knows all about the history of the Pokémon in the area. Ash meets this person while exploring an ancient ruin. The old person tells Ash stories about the origin of certain Alola Pokémon and gives him hints on how to find rare Pokémon or evolve his existing ones in unique ways.
He could meet Leon, the undefeated champion of Galar. Ash would surely be excited to battle such a strong trainer and learn from him.
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'll surely meet the Alola region's professors. Also, the local trial captains are very likely to cross paths with Ash. And of course, the regular Pokémon trainers in Alola.
He could meet an experienced trainer who becomes his mentor. This mentor could teach Ash some advanced Pokémon training techniques that he hasn't known before.
Obviously, Ash is one of the main characters. He is the protagonist who brings his experiences from other regions to Alola. His Pikachu is also a central character as always, being his trusty partner in battles and adventures. Then there are likely to be some new characters like his Alolan classmates or local trainers he befriends, for example, Kiawe or Lillie who play important roles in his Alolan journey.
He could bring some Johto Pokémon like Houndoom. Houndoom is a Dark - type Pokémon that would add a new element to Alola's Pokémon diversity. Its intimidating presence and strong battle capabilities would surely make it a popular addition among trainers in Alola.
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.
There are quite a few potential rivals. For example, Paul could be one. He has his own unique battling style and might be intrigued by Ash's Alola - champion ways. Then there's Trip. He could see Ash as a target to boost his own reputation. And in Alola itself, some of the more ambitious students from the Pokémon school who are aiming to be the best might also become rivals, constantly challenging Ash to prove they are worthy.