Some Pokemon fan communities on Reddit might also be a good source. People often share their fanfics there, and you might stumble upon some amazing meowth x pikachu stories.
One great meowth x pikachu fanfic is 'Adventures of Meowth and Pikachu'. It shows their unique friendship in a really interesting way. They go on quests together and face challenges side by side.
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 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 possible reason could be that Pikachu is separated from Ash. In many fanfictions, their bond is so strong that being apart would be extremely distressing for Pikachu, leading it to cry.
They could be in a fanfiction where they are forced to work together against a common enemy like a powerful trainer who wants to capture them both. Pikachu's speed and strong electric attacks combined with Pachirisu's cute charm and quick thinking can make for an interesting dynamic.
One great fanfiction idea could be a story where Pachirisu and Pikachu team up to save their forest home from a mysterious dark force. They use their unique electric powers in combination to fend off the threat and in the process, learn more about each other's strengths and weaknesses.
In some Pokemon fanfiction, Pikachu's ageing might be shown through physical changes like a slightly greying of its fur. This could symbolize the passage of time and the experiences it has had. For example, an older Pikachu might be a bit slower in battles but wiser in its strategies, having learned from countless encounters over the years.
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 common reason could be jealousy among other Pokemon. Some Pokemon might be jealous of Pikachu's closeness to Ash and its popularity. So, they might plot against it and end up killing it. Another reason could be a misunderstanding. For instance, a wild Pokemon might misinterpret Pikachu's actions as a threat and attack it fatally.