Perhaps a mysterious hermit who lives deep in the forest. The hermit has a special connection with Pokemon and can communicate with them in a unique way. Ash, being curious, would be drawn to this person. The hermit could teach Ash some new techniques for training and bonding with his Pokemon that he hadn't thought of before.
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.
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 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 could be a lone traveler who Ash meets while he is alone. This traveler might have a unique way of training Pokemon that Ash has never seen before and could teach him some new tricks.
There could be a wise old hermit who lives in a secluded forest that Ash meets during his solo travels. This hermit might have a deep knowledge of Pokémon lore and give Ash some valuable advice.
Potential characters might include a new group of Pokémon thieves that Ash has to deal with. Also, there could be a local Pokémon healer who becomes an important part of the story. There might be some young Pokémon enthusiasts who look up to Ash and join him on his adventures. Another possibility is a reclusive Pokémon trainer who has some rare Pokémon and Ash tries to learn from them.
He might meet an old Pokémon expert who gives him tips on hatching the egg.
He might meet Serena again. Since he has been to Alola and now travels to Kalos, their reunion could be an interesting plot point.
There could be a new trainer who has a very different style of training Pokémon compared to Ash. Maybe a young girl who uses music to communicate with her Pokémon. Another potential character could be an old Pokémon sage who has a vast knowledge of ancient Pokémon lore and can offer Ash some valuable advice on his journey.
There could be a new rival for Ash. This rival might have a very different approach to Pokemon training, like focusing more on the beauty and grace of Pokemon rather than just battle strength.
He might meet new Pokémon trainers in Alola. Maybe some young kids who are just starting their own Pokémon journey.