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.
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.
Ash joining Team Rocket could lead to some interesting character dynamics. He might start to question his own values. For instance, he's always been about friendship with Pokemon, but Team Rocket is more about using them for gain. This internal conflict could be a major part of the fanfic. Also, his relationship with Jessie, James, and Meowth would change drastically.
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.
Well, in the 'Ash and Misty joins Team Rocket fanfic', they could be brainwashed by Team Rocket's technology or some devious plan. Once they join, they might be given new uniforms and tasks. They could be involved in stealing Pikachu again, but this time from the other side. It would create a whole new dynamic in the Pokémon world, with their old allies now becoming their enemies.
Some of his more loyal Pokemon might try to resist the change. For example, Pikachu could use electric shocks to try and jolt Ash back to his senses. Pikachu has a strong bond with Ash and would be very confused and sad about his decision to join Team Rocket.
Ash might start to become more devious. He could learn some of Team Rocket's underhanded tactics, which would be a big change from his usually straightforward and honest self. But at the same time, he could also be a force for change within Team Rocket, slowly making them less evil.
One possible plot twist could be that Ash and Misty are actually undercover for a greater good. They pretend to join Team Rocket to expose their big plans from the inside. Another twist might be that they are forced to join because Team Rocket has kidnapped their Pokémon friends. So, they have to play along to save them.
Maybe there's a rogue Pokemon trainer. This trainer is jealous of Dawn's skills and the attention she gets. When Dawn joins Team Rocket, this trainer sees it as a chance to expose her as a fraud. He or she then tries to stop Dawn and Team Rocket's activities, thinking that they are up to no good, which they are, but the trainer has his own self - centered motives.