Well, one possibility could be that Meowth betrays Team Rocket. Maybe he gets tired of their constant failures and decides to side with Ash and his friends. He might see that they have a more positive and successful approach to their Pokemon journey.
Well, perhaps there could be a fanfiction where a powerful and mysterious Pokemon trainer captures both Meowth and Pikachu. The trainer forces them to battle each other for his amusement. But instead of fighting, they team up to escape. They use their unique skills, like Meowth's ability to speak human language to communicate with other Pokemon that can help them, and Pikachu's electric attacks to break through barriers. During their escape, they learn more about the trainer's dark secrets and also become closer friends.
One reason could be unrequited love. In the Pokemon world, Meowth has a soft spot for certain Pokemon, like the Skitty. If in a fanfiction, Skitty rejected Meowth's advances, it would make him sad. Another could be his longing for acceptance. Meowth is a part of Team Rocket, but often they don't fully appreciate his skills. He might feel sad and underappreciated in a fanfiction scenario. Also, memories of his past life on the streets, before joining Team Rocket, might haunt him and bring about sadness.
Well, it could be about a Meowth that discovers a special power that allows it to mega - evolve not once but twice. Maybe it goes on a journey to find a way to control this new - found power. Along the way, it meets other Pokemon and trainers who either help or hinder its progress.
In this fanfic, the plot could be that Meowth has a change of heart after an encounter with a powerful and kind Pokémon that Ash helps. This makes Meowth realize that Ash's way of treating Pokémon is better. So, he joins Ash. He could then use his unique skills, like his ability to talk, to help Ash in battles and communicate with other Pokémon more effectively.
However, over time, he could develop resilience. The experience of being abandoned might force him to become stronger, both physically and emotionally. He may learn to trust again, but it would be a slow process as he has to overcome the initial emotional trauma of abandonment.
One possibility could be that Meowth evolves after a deep emotional experience. Maybe it befriends a Pokémon that is in great danger and through the process of saving it, Meowth unlocks a new level of power within itself and evolves. This could lead to a whole new form with different abilities and a more noble appearance.
Meowth joining Ash in a fanfiction could also lead to some interesting character development for both. Ash might learn more about the Pokémon world from Meowth's perspective, especially the seedy side that Meowth has been a part of with Team Rocket. And Meowth could learn about friendship and loyalty in a new way, different from what he's experienced with Jessie and James.
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.
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.
Well, in the fanfiction, perhaps Ash catches Meowth to try and reform it. He might see that Meowth is not really a bad Pokémon at heart, just following the wrong crowd in Team Rocket. So he takes it under his wing, and they start to develop an unlikely friendship which changes both of them for the better.