Perhaps a local Alola elder who was initially supportive of Ash. But then, due to some misunderstanding or being influenced by an outside force, decides that Ash is not worthy of being the champion anymore. So, this elder starts a campaign against Ash, which makes Ash feel betrayed by someone he respected in the Alola community.
It could be one of Ash's Pokémon that gets corrupted in some way. Maybe by the influence of a dark power. This Pokémon then turns on Ash during a crucial battle, making Ash feel extremely betrayed. This would be a very emotional twist as Ash has such a strong bond with his Pokémon.
A fellow trainer in Alola could betray Ash. This trainer might be envious of Ash's success as a champion. He could secretly work with Team Skull or some other antagonists to undermine Ash. For instance, he might give false information to Ash about a battle location, leading Ash into a trap.
It could be Gary. Gary has always been a rival to Ash. Maybe in this fanfiction, Gary gets jealous of Ash's progress and decides to betray him by making false accusations to the Pokemon League about Ash using illegal training methods.
Maybe Team Rocket. They've been after Ash for a long time. If Ash comes back as a champion, they might see it as a threat and betray him in some way. They could pretend to be his friends at first and then stab him in the back to get what they want, like his powerful Pokémon or his championship title.
One possible plot could be that Ash is betrayed by his friends in the Alola region who are jealous of his champion status. They might spread false rumors about him, which leads to his isolation. For example, they could claim that he used unfair means to win the championship. This would make Ash feel betrayed as he thought they were his true friends.
His rival could be the one to betray him. Rivals are often jealous and competitive. If Ash's rival saw an opportunity to gain an advantage by betraying him, they might do it. For instance, they could lie about Ash to the Pokémon League, getting him disqualified from important battles.
It could be his rivals. For instance, Tracey or Paul might see Ash's potential in the new region as a threat and decide to betray him. They could try to sabotage his battles by giving false tips to his opponents or stealing his battle strategies.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
One aspect to consider in 'Ash, Champion of Alola' fanfiction is Ash's character growth. He has come a long way from his early days in the Pokémon journey. In this Alola setting, he has to adapt to new Pokémon, new training styles, and new friends. His victory as champion is not just about battles but also about his personal development and how he has influenced the Alola region. For example, his unique bond with his Pokémon in Alola might be different from his previous regions, which can be a great focus in fanfictions.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
It could be a rival trainer. Rival trainers are often very competitive in the Pokémon world. Maybe this rival is jealous of Ash and May's progress and decides to betray them to slow them down. They might spread false rumors about Ash and May, causing other trainers to be distrustful of them.
A great fanfiction idea could be Ash having to defend his championship title against a mysterious trainer from another region. This trainer has a unique set of Pokémon and a very different battle style from what Ash is used to in Alola. It would test Ash's skills and his ability to adapt as a champion.