In a fanfiction where Ash joins Team Rocket, it could lead to some really interesting storylines. Ash has a lot of knowledge about Pokémon training that Team Rocket lacks. He could potentially teach them new techniques, but at the same time, his moral compass would be constantly at odds with their devious ways. Maybe he joins them because he's somehow tricked or he has his own secret plan to take them down from the inside. It would be a great opportunity to explore different aspects of his character and how he adapts to this new situation.
The plot might involve a power struggle within Team Rocket. Ash and Misty could be brought in because they possess certain skills or knowledge. For example, Ash's bond with his Pokémon could be seen as a valuable asset. Misty's understanding of water - type Pokémon might also be desired. However, they soon realize that they are being used in a power play between different factions of Team Rocket, and they must decide which side to take or find a way to break free from the whole mess.
Well, if Ash joined Team Rocket, he'd probably bring a whole new dynamic. His skills as a trainer could be used in unexpected ways. Maybe he'd try to reform Team Rocket from the inside, using his kind heart and love for Pokémon to change their ways. He could also introduce new strategies for capturing Pokémon that are more ethical than Team Rocket's usual methods.
Well, one plot twist could be that Ash is actually a double agent. He pretends to join Team Rocket to gather information for the good guys. Maybe he discovers a big secret plan of Team Rocket, like they're planning to steal all the Pokeballs in a certain region. This would completely change the power dynamic and put Team Rocket on high alert.
Well, in such a fanfiction, Ash might start using Team Rocket's sneaky tactics. He could learn from them how to be more devious in catching Pokémon. For example, he might use their elaborate traps instead of his usual straightforward battles. This would be a huge change from his normal good - guy image.
Well, if Ash joins Team Rocket in a fanfic, it would be a huge shift in the Pokemon world. His friends, like Misty and Brock, would be shocked. They might try to rescue him. Ash, on the other hand, could initially be attracted to the idea of a new adventure with Team Rocket. He could see them as underdogs in a way, always failing but still trying. But as time goes on, he'd start to see the problems with their way of doing things.
Ash's character might become more conflicted. He has always been the hero, fighting against Team Rocket. But now, being part of them, he would question his own values. He might initially try to change Team Rocket from the inside, but as he faces more and more of their evil deeds, he would struggle between his loyalty to his new 'team' (out of a sense of obligation since he joined) and his true nature of being a Pokémon protector. Misty, on the other hand, would probably be more outspoken about her dislike for Team Rocket's actions and try to convince Ash to leave sooner.
Pikachu would probably be very confused at first. It's used to fighting against Team Rocket, not being on their side. It might resist and try to zap Ash to snap him out of it.
It would also have an impact on the other villainous teams in the Pokémon world. They might see Ash joining Team Rocket as a sign that they can also recruit more powerful trainers. This could lead to more alliances between the bad guys and more complex plots to steal Pokémon and cause trouble all around the Pokémon world.