One likely betrayer could be Serena. Maybe she has some hidden motives that are revealed when Ash becomes champion. For example, she might be jealous of his success and the attention he gets as champion, and so she betrays him.
One possible plot could be that Ash's so - called friends in the Kalos region turn on him out of jealousy. Maybe they can't handle the fact that he has become the champion. They start spreading false rumors about him, which makes the public turn against Ash. This betrayal could lead Ash to question his own journey and the relationships he has formed.
One possible scenario in this fanfiction could be that Ash has to overcome his toughest opponents yet in Unova. He might face trainers with unique and powerful Pokémon that he has never encountered before. His Pikachu would surely play a crucial role, using its speed and electric attacks to outwit opponents.
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.
The plot could involve Ash facing his toughest rivals from all the regions he has visited. He might have to overcome personal doubts and insecurities along the way. His Pokémon team could gain new powers or evolve at crucial moments. For example, his Charizard could return and play a key role in a battle. The final battle could be against a mysterious and powerful trainer who has a unique set of Pokémon. Ash would need to use all his experience and knowledge to win.
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 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.