He may simply be tired. The constant traveling, training, and battling can be exhausting both physically and mentally. Ash has been on this journey for a long time, and he could be at a point where he wants a more stable and less strenuous life. So, he stops being a Pokémon trainer and looks for a more relaxing and settled existence.
One reason could be that Ash has achieved all his major goals as a Pokémon trainer. He might have won all the important championships and feels there's nothing more to prove in that area. Another reason could be that he has had some kind of traumatic experience related to Pokémon training, like losing a very dear Pokémon in a battle that makes him not want to continue. Maybe he has also found a new passion, such as Pokémon research. He could have met some inspiring Pokémon researchers and decided to shift his focus from training Pokémon to studying them.
One reason could be that he gets tired of the constant traveling and battles. It's a very physically and mentally exhausting lifestyle. Another might be that he experiences a major loss, like losing his beloved Pokémon, which makes him lose his passion for training. Or perhaps he discovers a new interest that he finds more fulfilling than Pokémon training.
Some of his Pokémon might be very confused and sad. Pokémon like Pikachu, who has been with Ash for a long time, would probably miss the adventures and battles they had together. Pikachu might try to cheer Ash up and convince him to change his mind.
The story could focus on Ash exploring other aspects of the Pokémon world, like Pokémon research or Pokémon care. He might use his experience as a trainer to help those in need in different ways.
One great fanfiction about Ash as a legendary Pokémon trainer could involve him traveling to a new, mysterious region filled with rare and powerful Pokémon. He might start with his Pikachu, of course, and gradually build a team of Pokémon that are not only strong but also have unique abilities.
One reason could be a misunderstanding. Maybe Hermione does something that Harry misinterprets, like getting too involved with her studies and seeming to neglect their friendship. Another could be outside influence. For example, if there are other characters who spread false rumors about Hermione to Harry, it might drive a wedge between them. Or perhaps Harry starts to develop different values and beliefs that clash with Hermione's, leading to a breakdown in their friendship.
Ash can be seen as a bad trainer in fanfiction because of his poor communication with his Pokémon. Sometimes, he doesn't seem to fully understand what his Pokémon are trying to tell him. This lack of connection can lead to problems during battles or training sessions. His Pokémon might not be performing at their best because they are not being properly understood by their trainer.
Well, it could mean that his story would be a wild mix of different fan - created ideas. In fanfiction, various authors might have different takes on how he was raised, so he could be more of a rebel or have some strange powers. For example, some fanfictions might have him being raised by a rare and powerful Pokémon, which would give him a very different upbringing and perspective on the Pokémon world compared to the official story.
One possible reason could be that Izuku has grown up and wants to have a more formal relationship with Bakugou. As they progress in their hero training and development, Izuku might feel that the childhood nickname 'kacchan' no longer suits their new, more mature dynamic.
Some fanfictions focus on Ash's relationships with other trainers in the context of being a legendary trainer. He could form alliances with other powerful trainers, and together they face challenges like tournaments where the stakes are not just victory, but also the fate of the Pokémon world.
One possible reason could be that the fanfiction author wants to focus on other characters. Maybe they feel they've explored Naruto's character enough and want to give more spotlight to characters like Sasuke or Sakura. Another reason might be that the plot of the fanfiction takes a turn where Naruto's acting skills are no longer relevant or needed.