Another possibility is a new character. This new character could be someone who initially admires Ash but then becomes jealous of his success as a champion. They might feel that Ash doesn't deserve all the glory and betray him to take his place. This character could have some hidden motives or a dark past that drives them to betray Ash.
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.
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.
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.
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.
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.
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.