Ash may encounter a new character, perhaps a transfer student from another region who has unique Pokémon that Ash has never seen before. This new student could bring new techniques and knowledge about Pokémon training. Additionally, he might meet the school principal who has some strict rules about having Pokémon on school grounds, which would create an interesting conflict for Ash to deal with.
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.
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 some of his old friends again, like Brock or Misty. Maybe they will join him on his new journey through the regions.
He might meet Gary again. They could have a friendly chat or even a Pokémon battle.
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 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.
Gary could be one of his rivals. Gary has always been competitive with Ash. Another potential rival could be a new character introduced in the fanfiction, someone who is really good at Pokemon battles and wants to prove himself better than Ash.
He might meet Serena again. Since he has been to Alola and now travels to Kalos, their reunion could be an interesting plot point.
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 Pikachu first. Since Pikachu is his closest and most loyal Pokemon friend, it would be likely that Pikachu would be by his side when he wakes up. Pikachu could be really confused at first but still show concern for Ash in his new state.