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.
One idea could be that Ash feels betrayed by his friends because they always overshadow his achievements. So, in a fit of anger and disappointment, he decides to join Team Rocket. He might think that with their resources, he can finally become a more powerful trainer on his own terms.
The plot could revolve around Ash's initial confusion and conflict. He might join Team Rocket thinking he can change them, but soon realizes it's not that easy. His loyalty to his old friends would be tested as he gets drawn deeper into Team Rocket's plans.
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 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.
He might become more cautious. Usually, Ash is quite bold and rushes into things. But if he's in a Team Rocket - centered fanfiction, he'd likely be more careful as he's constantly surrounded by danger. For example, he'd think twice before challenging a new Pokemon or entering an unknown area.
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 could lead to all sorts of adventures. Maybe Ash would learn new skills related to flower care and use them in battles.
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.
They could also change Team Rocket for the better. Maybe they would introduce a new sense of honor or fairness. Ash's determination and Misty's feistiness could influence the other members to be less about pure evil and more about having fun in their villainous deeds. In a fanfiction, this could lead to some really interesting character development for both Ash, Misty and the Team Rocket crew.
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.