Some of the gym leaders could also be involved. Maybe they feel threatened by Ash's new - found power as a champion. They might think that Ash will change the way the Pokemon battling world works and in order to stop that, they betray him. They could spread lies about Ash misusing his Pokemon or something similar.
One possible plot could be that Ash's so - called friends turn against him out of jealousy when he becomes a champion. They might spread false rumors about him, making him feel betrayed.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
In an Ash betrayed champion fanfiction, perhaps the champion has been secretly working with a villainous group. The champion lures Ash into a trap under the pretense of a friendly battle. Ash, being trusting, falls for it. He then has to find a way to escape and get revenge. He might meet some new allies along the way who help him see the champion's real motives.
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.
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.
Ash could face some tough challenges in this fanfiction. He might have to train in harsh conditions to become a champion. For instance, training on a snowy mountain where Pokemon are stronger due to the environment. His team would need to adapt and grow. Maybe he gets a new Pokemon that is a bit of a troublemaker at first but later becomes a key player in his battles.
In this kind of fanfiction, Ash could have many new adventures on his journey to become a champion. Maybe he discovers a new type of Pokemon that gives him an edge in battles. For example, a Pokemon with a unique ability that can counter the most powerful opponents. He might also form new friendships along the way that help him in his quest, like teaming up with a trainer who has knowledge of ancient Pokemon battle techniques.
One possible analysis could be that in such a fanfiction, the idea of Ash being betrayed might be a way to create drama. It could involve his friends or companions in the harem turning against him for various reasons like jealousy or misinformation.
One possible plot could be that Ash returns as a champion but is betrayed by his former friends who are jealous of his new status. They might spread false rumors about him, causing him to lose the support of the people. Ash then has to prove his worth all over again and show that he is still the same person at heart.