The plot could involve Ash's friends trying to rescue him from Team Galactic. They would suspect that something is wrong when Ash starts acting differently. His Pikachu, being very perceptive, might also sense the change in Ash and try to lead the others to find him. Meanwhile, within Team Galactic, Ash could be in a position where he has to make a choice between following their dark path or finding a way to expose their true intentions and escape.
One possibility could be that Ash is initially deceived by Team Galactic's false promises of creating a better world for Pokemon. He might be drawn in by their grand talk about power and a new order. For example, they could convince him that their plans for using the power of the legendary Pokemon would benefit all Pokemon in the long run. However, as he spends more time with them, he starts to notice their more sinister motives, like their disregard for the balance of nature and the harm they are actually causing to Pokemon habitats.
Ash joining Team Galactic in fanfiction could mean a complete shift in his character arc. He might be forced to do things against his will at first. But as time goes on, he could use his knowledge of Pokemon to gain an upper hand within the team. For example, he could use his bond with his Pokemon to resist the team's orders that are harmful to Pokemon.
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.
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.
In a 'pokemon dawn joins team galactic' fanfiction, the plot could involve Dawn being lured by Team Galactic's false promises of power. Maybe they convince her that their goals for the Pokemon world are noble at first. She might start to question her own values and the way she has been training Pokemon. However, as she delves deeper into Team Galactic, she could discover their true evil intentions and then have to struggle to break free and redeem herself.
Well, if May joins Team Galactic in the fanfic, she might be used as a pawn at first. Team Galactic could see her as a way to get to other trainers or Pokemon. However, May is a strong - willed character. So she could start to secretly undermine Team Galactic from within. For example, she might feed false information to them or help other Pokemon escape their clutches. Eventually, she would probably lead a rebellion within the team to expose their wrongdoings.
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 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.
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.
Obviously, Team Galactic itself would be the main antagonists. Their leaders, like Cyrus, would play a big role. Cyrus has his own extreme ideas about remaking the world, which goes against the harmony of the Pokemon world. He would see Dawn as a tool to achieve his goals, and his cold and calculating nature would make him a formidable foe for Dawn in the fanfiction.
If May joins Team Galactic in the fanfic, her character would go through significant changes. At the start, she might be curious and excited about being part of something new. But as she realizes the true nature of Team Galactic, she would experience a lot of guilt for being associated with them. This guilt would drive her to try and make amends. She would become more determined and勇敢 (brave in Chinese, for lack of a better English equivalent in this context). Her relationship with her Pokemon would also change, becoming stronger as they support her in her fight against Team Galactic.