He might meet some of his old friends again, like Brock or Misty. Maybe they will join him on his new journey through the regions.
In this fanfiction, Ash may meet trainers who have a unique connection to the regions he is revisiting. For instance, a trainer who has been researching a particular Pokémon species native to a region Ash is going through. He could also meet descendants of old Pokémon trainers or gym leaders. There might be a chance that he meets some Pokémon that have a special bond with certain areas of the regions, and through them, he gets to know more about the land and its secrets.
Ash might meet a strict teacher who has no idea about Pokémon at first but gradually becomes interested in them. He could also meet a rival in school who challenges him not only in academics but also in Pokémon battles.
In this fanfiction, Ash might encounter new Pokémon species in the regions he revisits. He could also form new friendships or rivalries with trainers he meets along the way.
Pikachu, of course. It's always been Ash's closest partner.
He might meet new Pokémon trainers in Alola. Maybe some young kids who are just starting their own Pokémon journey.
He might meet Gary again. They could have a friendly chat or even a Pokémon battle.
Perhaps Ash is on a quest to complete a special Pokémon research project this time. He travels through the regions, interviewing different trainers and Pokémon experts. He might get involved in some local Pokémon traditions and competitions that he hadn't experienced before. His Pikachu will surely be by his side, and they might develop new battle strategies against the diverse Pokémon they encounter.
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 an experienced trainer who becomes his mentor. This mentor could teach Ash some advanced Pokémon training techniques that he hasn't known before.
He might meet an old Pokémon expert who gives him tips on hatching the egg.
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.