It could be that a new and powerful trainer befriends Ash at first but then betrays him to take over his champion position. This new trainer might use underhanded tactics, like stealing Ash's Pokemon strategies and using them against him, leaving Ash feeling extremely let down.
Perhaps in the fanfiction, the Pokemon League officials betray Ash. They might try to strip him of his champion title unjustly due to some hidden agendas, like wanting to promote a different trainer for commercial reasons. This would be a huge betrayal for Ash who has worked so hard to reach the champion status.
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.
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.
Perhaps a new and powerful trainer arrives in the story. This trainer manages to deceive Ash's harem members, making them think that Ash is a hindrance to their growth as trainers. As a result, they side with the new trainer and betray Ash, leaving him feeling confused and alone.
One interesting plotline could be Ash facing a new and extremely powerful legendary Pokemon that challenges his skills as a champion in ways he's never experienced before. This could lead to him having to develop new battle strategies.
One common plot is that Ash's friends or some of the female characters in his harem turn against him due to a misunderstanding. Maybe they think he has some ulterior motives or has betrayed their trust in some way. Another plot could be a new character coming in and manipulating the situation to make it seem like Ash is the bad guy.
Well, perhaps in the fanfiction, Ash gets influenced by a powerful and devious trainer. This trainer makes Ash believe that May is holding him back in his Pokémon journey. So, Ash starts to distance himself from May, not realizing he's being manipulated. May, on the other hand, is left heartbroken and feeling betrayed as she doesn't understand why Ash is suddenly so cold towards her. It could also be that Ash enters a Pokémon tournament without May, even though they had promised to do it together all along.
One interesting plot could be Ash forming a unique bond with each Pokémon in his harem. For example, he might have special training sessions with different Pokémon that are tailored to their individual strengths and weaknesses.
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.
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.
The battles. In a good 'pokemon ash champion fanfic', the battles are intense and well - described. For example, Ash using unique strategies to defeat his opponents makes it exciting.
A possible plot could be that Ash gets lost in a spooky area where Fantina is known to reside. She takes him under her wing and shows him the secrets of the area. Along the way, they train their Pokemon together and Ash learns new battle techniques from Fantina. This encounter could also lead to Ash becoming more confident in dealing with Ghost - type Pokemon, which Fantina is an expert in.