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.
By showing his lack of interest in things he usually loves. In normal situations, Meowth is quite interested in shiny objects or food. So, in a sad fanfiction, the writer could depict him ignoring a pile of gold coins or a plate of his favorite food. The writer can also create a contrast in his behavior. For instance, Meowth is usually talkative, but when sad, he becomes quiet and withdrawn, only meowing softly when necessary. This change in behavior will clearly show his sadness to the readers.
The writer can use the environment to enhance Meowth's sadness. If the story is set in a rainy day, with Meowth sitting alone under a tree, getting wet, it adds to the melancholic mood. Another way is to have other characters notice his sadness. For example, Jessie and James might comment on how Meowth is not his usual self. Their concern and confusion about his state can also help in highlighting Meowth's sadness more effectively in the fanfiction.
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.
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.
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.
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.
In a fanfic about Meowth evolving, it could be that a powerful enemy forces Meowth to adapt and evolve in order to protect its friends. This evolution might give it new abilities and a different appearance, perhaps more intimidating to foes.
One sad Meowth story could be that Meowth was in love with a female Meowth. But she rejected him because he could speak human language which made him different. He was left all alone, feeling heartbroken and out of place in his own kind's community.
The storyline would likely become more complex. With Meowth around, there would be a new dynamic. His loyalty would be split between Ash and his old Team Rocket pals at first. This internal conflict could drive parts of the story.
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.
In the fanfic, the Mega Mega Meowth could be the center of a mystery. No one really knows how it came to be or what its full capabilities are. Trainers from all over might be trying to study it or capture it. This would create a lot of tension and excitement in the story. The author could also explore how the Pokemon world reacts to such a powerful and unusual creature. Maybe some see it as a threat and some see it as a symbol of hope for new power and evolution in the Pokemon world.