Another reason could be a difference in ideology. Ash might have developed a new view on how to protect and develop Alola that goes against the traditional ways of the region. His new ideas could be seen as a betrayal by those who are more conservative in their approach to Alola's well - being.
It could also be due to some external influence. Perhaps an evil organization has manipulated Ash in some way, forcing him to act against Alola. They might have used mind control or blackmailed him with something he holds dear, like the safety of his Pokémon.
In some fanfictions, Arceus might be bound by some ancient rules that Ash unknowingly breaks. These rules are so important to Arceus that it feels compelled to betray Ash. For example, if there's a rule about not interfering with a certain type of Pokémon evolution and Ash does it accidentally, Arceus could see it as a major transgression and take action against him.
Maybe the loss of a very dear Pokémon friend. If something bad happened to a Pokémon he was extremely close to, like it getting seriously injured or lost forever, it could be so devastating that he loses the will to continue his journey and thus gives up.
In some fanfictions, Percy might betray Natasha due to his own ambition. He could see an opportunity to gain more power or status by siding against her. For example, if there's a power struggle within their group or world, and he thinks that by betraying Natasha, he can get on the winning side and be rewarded. It could also be a case of jealousy. If Natasha has always been in the spotlight or has more skills and abilities, Percy might feel overshadowed and his jealousy could lead to betrayal.
One possible plotline could be that Ash is influenced by a powerful and evil force. This force convinces Ash that Cynthia is somehow a threat to his ultimate goal, like becoming the greatest Pokémon trainer. So, he betrays her by stealing a crucial Pokémon of hers during a big tournament. This would lead to a lot of drama and conflict as Cynthia would be shocked and heartbroken, and Ash would start to realize the error of his ways as the evil force begins to control him more.
It could be because of a power struggle. Suppose Naruto discovers a new, extremely powerful power within himself, but the village elders or some members of Team 7 are afraid of it. They try to suppress him. Feeling cornered and betrayed first, Naruto could then turn against them. He might believe that he has to protect this new power and use it for what he thinks is right, even if it means going against his old team.
Perhaps Natsu feels betrayed by the guild first. There could be a situation where he believes the guild has turned its back on one of his closest friends or on a principle he holds dear. This sense of being let down by the guild might lead him to take extreme actions that seem like betrayal. It could also be related to a power struggle. If there's a new power within the guild that Natsu sees as a threat to the guild's original values, he might try to oppose it in a way that looks like betrayal from the outside.
Perhaps in the fanfic, Ash betrays Tyson because of a misunderstanding. There could be a false rumor spread about Tyson being involved in some unethical Pokémon training methods. Ash, believing the rumor without fully investigating, turns his back on Tyson. Later, when he realizes the truth, he has to work hard to regain Tyson's trust.
Well, in some fanfictions, Ash might fall in love with a fellow Pokémon coordinator in Alola. They could start off as rivals, constantly trying to outdo each other in Pokémon showcases. But as they see the dedication and kindness in each other, their relationship changes. They might start training together, sharing tips on how to make their Pokémon look more beautiful and perform better. This mutual respect and admiration could easily turn into love. Or perhaps Ash meets a mysterious girl who has a special bond with a rare Pokémon. Ash is intrigued by this bond and wants to learn more about it. As he delves deeper, he falls for her.
One possible storyline could be Ash's journey after becoming the Alola champion. He might travel to other regions to share his Alola - inspired battle strategies. Another could be him mentoring new trainers in Alola, passing on his knowledge and experiences. And perhaps he could face challenges from trainers who want to take his champion title.
It's possible that Greninja's return is part of a prophecy in the Alola region. The local people might see Greninja as a sign of great change or a hero. So Ash and Greninja have to work together not just to battle but also to fulfill this prophecy, which could involve saving the Alola region's natural balance or something similar.