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.
Another idea is that a new trainer comes to the Alola region and befriends Ash. But this trainer has ulterior motives. He or she gets close to Ash to steal his Pokémon battle strategies. Once they have what they need, they turn on Ash and use the stolen strategies against him in battles, making Ash feel betrayed by someone he considered a friend.
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.
In such a fanfiction, perhaps Ash could be betrayed by his friends in Alola. Maybe they misunderstand his actions or are influenced by some false information. For example, they might think Ash is secretly working for an enemy, which leads to the betrayal.
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.
In such a fanfiction, perhaps Ash is betrayed by his so - called friends in his original region. They might steal his Pokemon or exclude him from important battles. Moving to Alola, he could start a new journey. He might meet new Pokemon and friends like Lillie and Hau. He'll have to regain his confidence and prove himself all over again in the Alola region.
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.
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.
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.
There are quite a few potential rivals. For example, Paul could be one. He has his own unique battling style and might be intrigued by Ash's Alola - champion ways. Then there's Trip. He could see Ash as a target to boost his own reputation. And in Alola itself, some of the more ambitious students from the Pokémon school who are aiming to be the best might also become rivals, constantly challenging Ash to prove they are worthy.
One great 'Pokemon Ash Alola Champion' fanfiction is 'The Alola Ascent'. It beautifully details Ash's journey to becoming champion in Alola, with unique character developments and exciting battles. Another is 'Ash's Alola Legacy' which focuses on how he changes the perception of being a champion in the Alola region. There are also many on fanfiction.net that you can explore.
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.