One common theme could be Ash's feelings of isolation and despair. After being betrayed, he would likely feel very alone and question his past relationships.
I'm not sure of specific fanfics on this topic offhand. You could try searching on popular fanfic platforms like Archive of Our Own. Just type in the keywords 'ash gets betrayed by most of his friends' in the search bar and browse through the results.
One possible plot could be that Naruto discovers his friends have been secretly working with an enemy. They might have been promised power or something valuable in return for betraying Naruto. Maybe they spread false rumors about Naruto, turning the village against him.
He might be initially shocked. Harry has always trusted his friends, so when the betrayal happens, it would be like a bolt from the blue. He could stand there speechless for a moment, unable to believe what has just occurred.
Well, often in such a 'ash gets betrayed fanfiction', the story might start with a seemingly normal day for Ash. Then, out of the blue, someone he trusts deeply, like Pikachu or one of his human friends, does something that he perceives as a betrayal. This could be something as simple as not believing in him during a crucial battle or siding with an enemy for a reason that's not clear to Ash at first. It can create a lot of emotional turmoil for Ash and make for a very engaging read.
Ash would probably be devastated. He would feel like his world has been turned upside down. At first, he might not believe it and think it's all a big mistake. As the reality sinks in, he could start to isolate himself from others, even from his Pokemon for a while. However, his strong will would eventually kick in. He would use this as motivation to become stronger, not just in his Pokemon battles but also in his ability to face challenges alone, without relying on his so - called friends.
Naruto would likely become more distrustful at first. He's always been so open and trusting of his friends, so this betrayal would shatter that. He might become more withdrawn, constantly second - guessing people's motives.
One possible reason for Ash getting betrayed in the fanfiction could be misinformation. A villain might spread false rumors about Ash, making his friends believe he has done something wrong. His friends, without verifying the truth, turn against him. It's a sad situation where lack of communication and trust plays a big role.
Jealousy could be one reason. Maybe his friends are jealous of his achievements as a Pokémon trainer. Another reason could be that they are influenced by some external forces, like an evil organization that convinces them to betray Ash.