Well, in fanfiction, Ash might stop being a Pokémon trainer if he realizes that the Pokémon world has some dark sides that he can't tolerate. For example, if he sees too much exploitation of Pokémon in some areas. Also, he could meet someone who convinces him that there are other important things to do in life. Maybe he falls in love and his partner wants him to settle down and give up the dangerous life of a trainer.
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.
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 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.
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.
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.
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.
A Darkrai could be in Ash's secret team. Darkrai is known for its ability to create nightmares, which could be used to disorient opponents in battle. It's a very powerful and mysterious Pokémon that would add an element of surprise to Ash's team.
One reason could be his sense of justice. Ash has always been about doing what's right in the Pokémon world, so saving all the legendaries would be a natural extension of that. Another reason might be his love for Pokémon in general. He wants to protect all Pokémon, and legendaries are no exception.
Another reason might be related to their different goals. Ash is mainly focused on becoming a Pokémon Master, while Misty might have her own goals that sometimes conflict with Ash's journey. This could lead to friction between them and eventually make Ash develop negative feelings towards her.