One possible storyline could be that Meowth helps Ash find a rare Pokémon. Since Meowth has his own network among Pokémon, he might hear rumors about a very special Pokémon. He then takes Ash to the location, and they have an epic adventure trying to catch it. Another storyline could be that Meowth and Ash team up to take down a new evil organization that's causing trouble in the Pokémon world.
If Meowth joins Ash in fanfiction, it could lead to some really interesting character dynamics. Meowth has been part of Team Rocket for a long time, so there would be a lot of room for him to change and grow. He could teach Ash some of the sneaky tricks he learned from Team Rocket, but in a more positive way. Maybe they'd form a new team and compete in different Pokémon competitions.
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.
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.
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.
There could be a storyline where Spider - Man uses his scientific knowledge to upgrade the Avengers' technology. Or perhaps a story where he has to choose between his loyalty to the Avengers and protecting his own neighborhood. This internal conflict could add depth to the fanfiction. Also, he might team up with Black Widow for a stealth mission where his agility and her skills combine for a successful operation.
One possible storyline is that they meet while investigating a strange phenomenon in the Pokémon world. They join forces to figure out what's going on and end up uncovering a plot by an evil team. Another could be that Misty has become a Pokémon trainer herself and challenges Ash to a battle when they meet. Or they could meet at a Pokémon sanctuary where they both have the same goal of helping injured Pokémon.
One possible storyline could be that Meowth is abandoned by a trainer who has caught a stronger Pokemon and no longer has use for him. Meowth then has to fend for himself on the streets, facing other wild Pokemon and trying to find food. He might also start to question his self - worth after being discarded so easily.
A great 'ash and meowth fanfiction' often has a strong emotional connection between the two characters. It might show their mutual respect growing over time. For example, they could start as enemies but gradually become friends. Also, an interesting plot that takes them out of their comfort zones, like exploring a new region or facing a powerful new enemy.
His personality might become more positive. Instead of being a bit of a schemer like he was with Team Rocket, he could become more helpful and honest. He might start to care more about others' feelings.
One storyline could be that Naruto joins Kabuto to gain access to some special power or knowledge that Kabuto claims to possess. Naruto believes it can help him protect the village better.