His other Pokemon like Charizard might outright refuse to follow Ash if he joins Team Rocket. Charizard has a strong sense of pride and loyalty, and joining the bad guys goes against everything it stands for. It could lead to some epic battles between Ash and his disobedient Pokemon.
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.
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.
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.
It could lead to some really unexpected and exciting plot twists. Ash's alignment with Team Rocket would completely change the dynamics of the story.
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 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.
If Team Rocket turns Ash into a Pokemon, it could lead to a really interesting story. Ash would have to experience the world from a Pokemon's perspective. He'd understand their struggles, like being caught and trained. This could also be a way for him to better understand his own Pokemon and their bond with him. And it would be a wild adventure as he tries to find a way to turn back to his human form.
Pikachu would likely be very confused and resistant. Pikachu has a strong bond with Ash and knows his true nature. It would probably try to shock Ash out of whatever trance or false belief he has from joining Team Galactic. For example, when Ash starts to follow Team Galactic's orders that go against his usual principles, Pikachu might refuse to cooperate and try to show Ash that he's on the wrong path.
One possibility is that Ash joins Team Galactic under some sort of misunderstanding. Maybe he thinks they are a group with good intentions at first. Once he realizes their true nature, he has to figure out a way to escape or stop them. This could create a lot of tension and action - packed scenes in the fanfiction.
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, 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.