Well, it could make for a really interesting fanfiction. Dawn joining Team Rocket would change the whole dynamic. Her Pokémon might not be too happy about it at first. She could bring new ideas to Team Rocket, since she has a different way of thinking compared to Jessie, James and Meowth. However, it's also possible that she would secretly be working against Team Rocket from the inside, trying to take them down while pretending to be one of them.
In fanfiction, if Dawn joins Team Rocket, it would shock her friends and fans alike. Her motives for joining could vary. It could be a case of misinformation where she thinks Team Rocket is something it's not. Once in Team Rocket, she might find herself in a moral dilemma. She could start to see the error of her ways and try to reform Team Rocket from within. Or she could be a full - on villain, using her skills as a trainer to further Team Rocket's evil plans, which would create a lot of conflict in the Pokémon world.
One possible storyline is that Dawn joins Team Rocket thinking it's a legitimate Pokémon training organization at first. But as she gets deeper into it, she discovers their true evil nature. Another could be that she is brainwashed by Team Rocket into joining them. Her friends then have to find a way to break the brainwashing. A third storyline could be that she joins willingly because she has some sort of vendetta against the Pokémon League and sees Team Rocket as a way to get revenge.
Well, if Misty joins Team Rocket in fanfiction, it would be a huge shock to the other characters. Her friends like Ash would be completely bewildered. Maybe she's been tricked into it somehow, like being deceived by Team Rocket's false promises. This could lead to some really interesting plot twists. For example, she might be used to set traps for Ash and the others, but deep down still have some good in her that causes internal conflict within Team Rocket.
Well, if Serena joins Team Rocket in a fanfiction, she might bring a new dynamic. Maybe she's forced to join at first. She could use her skills in battles and her knowledge of Pokemon to help Team Rocket in their schemes, but secretly she could be planning to take them down from the inside.
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.
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.
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.
One possible plot could be that Dawn, for some reason, gets disillusioned with her regular Pokemon journey. Maybe she has a series of losses that make her question her methods. Team Rocket, seeing an opportunity, convinces her that they have a better way to train and use Pokemon. She then joins them, initially helping with their small-time schemes, but gradually realizes that their actions are unethical and has to find a way to break free.
One thing that would happen is that his Pokémon battles would take on a whole new meaning. Instead of battling for the sake of becoming a Pokémon master, he might be battling to further Team Rocket's goals. His Pokémon would have to adjust to this new purpose. Also, Ash would probably have to wear Team Rocket's uniform, which would be a really strange sight for fans who are used to seeing him in his normal get - up.
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, 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.