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.
Perhaps in the fanfiction world, there is a special item that causes Meowth to evolve. This item could be a rare crystal found in a mysterious cave. Meowth, being the curious Pokémon it is, ventures into the cave and discovers this crystal. When it touches the crystal, it is bathed in a bright light and evolves. The new evolved form of Meowth might have a shiny coat of fur and the ability to speak more clearly to humans, as well as having stronger attack and defense stats.
In a fanfiction, Meowth might evolve as a result of intense training. Picture Meowth constantly challenging stronger Pokémon in battles, getting beaten down but never giving up. Each battle teaches it new strategies and toughens it up. Eventually, this unwavering determination causes it to evolve. It could gain enhanced speed and the ability to use psychic - type moves, which would be really cool.
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.
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.
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.
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.
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.
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.
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.
It could also revolve around a genetic experiment gone wrong that accidentally creates a new form of evolution for some Pokemon. These newly evolved Pokemon are initially feared but eventually prove to be helpful in a crisis situation, teaching the humans and other Pokemon the importance of acceptance and adaptability.
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.