In this kind of fanfiction, Clemont might betray Ash for the sake of scientific discovery. He could think that by sacrificing Ash's trust, he can gain access to some rare Pokemon or a powerful energy source that would further his research. He may feel that his own goals are more important than his friendship with Ash at that moment.
One possible plot could be that Clemont is brainwashed by an evil organization. They make him believe that Ash is a threat to the Pokemon world. So, he starts to act against Ash, like revealing Ash's battle strategies to his opponents or stealing important items from him. This would create a lot of drama and conflict in the story, as Ash would be completely shocked by his friend's sudden change of behavior.
Well, perhaps Clemont betrays Ash because he becomes envious of Ash's success. Maybe he gets influenced by some dark force in the Pokemon world that convinces him to steal Ash's Pokemon or to lead him into a dangerous trap.
It could be Team Rocket. They might have some sort of hold over Clemont, forcing him to betray Ash. Team Rocket has always been after Ash's Pikachu, and using Clemont to get to Ash would be a devious plan for them.
One possible plot could be that within the fanfiction world, there is a new character introduced who seems to be a great ally at first. This character gains the trust of Ash and Brock over time. However, this new character has a hidden agenda. They might be from a rival region and their goal is to undermine Ash and Brock's reputation in the Pokemon world. So they start spreading false rumors about them and turn other trainers against them, thus betraying their new - found friendship.
In such a fanfiction, the plot could involve Ash being betrayed by his long - time friends. Maybe they envy his success in the Pokemon world. For example, Gary could spread false rumors about Ash in the new region, causing the local trainers to distrust Ash. This would make Ash's journey in the new region much more difficult as he has to not only deal with new Pokemon and challenges but also regain his reputation.
It could be that Lillie betrays Ash because she believes it's the only way to achieve a greater good in the Pokémon world. She might think that Ash's methods are too dangerous or not in line with what she believes is right for the future of Pokémon and humans alike. This would create a complex conflict between their ideologies.
Well, in many such fanfictions, it could involve Ash being betrayed by Clair perhaps in a Pokémon battle strategy. Maybe Clair teams up with an unexpected rival against Ash, or she might break a promise regarding Pokémon training or a joint adventure.
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.
In such a fanfiction, Ash might become extremely isolated. He could lose faith in his Pokémon and himself. Maybe he'd leave the Pokémon world in despair, feeling that all his efforts were in vain. Without the support of his friends and Pokémon, his once - strong will to be a Pokémon Master could crumble completely.
One interesting idea for the fanfiction is that May betrays Ash to protect her family. An evil group threatens her family and the only way to save them is to turn against Ash. This would create a complex situation where May is torn between her loyalty to Ash and her love for her family. Ash, on the other hand, would be completely blindsided by her sudden change in behavior.
It could be that Mew has some sort of special power or knowledge that certain groups in the Pokémon world are after. They see Ash as an obstacle to getting what they want from Mew. So, they scheme to make everyone think Ash is untrustworthy. His friends, not knowing the truth, end up betraying him. Ash then has to fight against all odds to protect Mew and regain the trust of his friends.
Friendship is a very common theme. Since Clemont and Ash are friends in the Pokemon universe, many fanfictions explore how their friendship grows and endures through different adventures. Another theme is growth, both in terms of their skills as Pokemon trainers and as individuals. They might learn new battle techniques or become more mature in handling difficult situations.