Well, there could be new characters created specifically for this fanfiction. For example, a mysterious trainer who has some knowledge about Ash's chosen - one destiny and tries to help or hinder him. There might also be a Pokémon that has a special connection to Ash because of his chosen - one status. And of course, Team Rocket would still be around, constantly trying to get in the way of Ash's journey as the chosen one.
In this fanfiction, Gary could be an important character. Since he has always been in competition with Ash, he might have mixed feelings about Ash being the chosen one. Serena could also be a main character. She might be inspired by Ash's chosen - one journey and decide to follow him, offering her own skills and support. Professor Oak would likely be involved as well, guiding Ash with his vast knowledge of Pokémon.
His Pokémon are central, as they are always by his side. Pikachu would be a key figure in any story related to Ash. Then there are his human friends. Brock could be involved in cooking for Ash, which might contribute to the weight gain. Misty could be there to comment on Ash's appearance and give her opinions. And in some fanfictions, Nurse Joy or Professor Oak might also be included to provide medical or wise advice regarding Ash's situation.
Brock could also be a main character. Since he has knowledge about Pokémon care and breeding, he could be Ash's colleague in the fanfiction. They could work together on projects related to Pokémon health and nutrition.
Misty and Brock could be important characters as well. They have been with Ash for a long time. They might be deeply affected by Ash's supposed death and be the ones leading the search to find out what really happened.
A young girl named Mia might be a character too. She is also a Glacion fan and is very competitive. She challenges Ash to various Glacion - related challenges, like a Glacion knowledge quiz or a race to find a hidden Glacion item in the forest.
Well, Pikachu could still be a main character. Even if Ash hates everyone, Pikachu might be the one trying to bring back the old Ash. Maybe Team Rocket could also be main characters. They are always around Ash, and his new - found hatred could make their interactions even more interesting. They might be confused by his change or even try to take advantage of it.
May could be one of the main characters. She could join Ash on his journey and share her knowledge about Hoenn's Pokemon with him. Another could be Brock, still tagging along to offer his expertise in Pokemon care.
Team Rocket might be main characters as well. They could be constantly trying to capture Ash or his Pokemon because of his special status. Their bumbling attempts to take advantage of Ash's link to Arceus could add a lot of comical and action - filled moments to the fanfiction. And of course, Arceus itself could be a major character, guiding Ash in some way or interacting with him directly.
Latios would obviously be a key character. Also, Pikachu, Ash's loyal Pokémon partner, would be there. Maybe Misty or Brock could also be part of the story, with them either supporting or being skeptical of Ash's newfound admiration for Latios.
Obviously, the Legendary Pokemon would be a main character. It could be a Pokemon like Mewtwo, who is often depicted as having a complex personality. It might start off as a mysterious and powerful entity that Ash has to befriend or battle against.
Well, perhaps a plotline where Ash discovers that being the chosen one means he has a connection to an ancient Pokémon prophecy. This prophecy might involve saving the Pokémon world from a looming disaster. Along the way, he might gain new Pokémon partners that are specifically destined to help him in this grand task. He could also face challenges from other trainers who are jealous of his chosen - one status.