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.
In a fanfic, Meowth's evolution might be a result of a magical spell gone wrong. Instead of evolving into a normal form, it becomes a hybrid of different Pokemon types. This new form gives it strange and wonderful abilities, like being able to turn invisible or create illusions. But it also has to deal with the confusion of its new identity and how other Pokemon and humans react to it.
If Meowth evolves, its relationships with other Pokémon could change significantly. For example, Pokémon that used to bully it might now be intimidated by its new form and power. Meowth's old friends, on the other hand, could be proud of it. They might look up to it for protection now. For instance, Pikachu, who was once an equal to Meowth in a sense, might now see Meowth as a stronger ally and be more likely to team up with it in battles.
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 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.
A writer can use Meowth's body language. For example, they can describe his drooping ears, his slow and slouched walk, and his tail hanging low. This physical appearance immediately gives the readers an idea of his sadness. Also, the use of simple dialogue like 'Meowth meow (I'm so sad)' can be effective.
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.
Another possibility is a rival trainer who has a Pokemon that is weak against Raichu. This trainer might try to prevent Pikachu from evolving to keep their own Pokemon's advantage in battles. They could try to steal Pikachu's food or training resources to slow down its growth and evolution.
One possible plot could be that Pikachu is exposed to a rare form of electric energy in a thunderstorm that's charged with some mysterious power. This could start the evolution process. It might struggle at first, not understanding the changes its body is going through. As it evolves, it could gain new abilities like enhanced speed and a more powerful electric attack. And it might have to face new challenges in battles, as opponents are now more cautious of it.
It could be Giovanni. If Giovanni gives an order that Meowth finds completely unacceptable, like harming innocent Pokemon for no good reason. Meowth, having a soft heart deep down, might not be able to go along with it and thus betray Team Rocket.