One possible motive could be that Team Rocket has constantly been causing trouble for Ash and his friends. They've been trying to steal his Pokémon time and time again, which might have finally pushed Ash over the edge.
Yes, in some Team Rocket fanfictions, Ash can be depicted in various ways that might involve him having some sort of connection to Pokémon in a different light. Maybe he could be experimented on by Team Rocket to gain Pokémon - like abilities, or be transformed into a Pokémon - like being in a very creative and out - of - the - box fanfiction concept.
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.
Another plotline might involve Ash teaming up with some of the Team Rocket members, perhaps reluctantly at first. They could be forced to work together due to a common threat to both the good guys and the bad guys in the Pokemon universe. This would lead to some interesting character developments for Ash as he has to deal with the not - so - good nature of Team Rocket.
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 role is that he could be a spy. Since he was turned into a Pokémon, Team Rocket might not suspect his true identity and he could gather information about their evil plans. For example, he could overhear conversations between Team Rocket members that he couldn't have accessed as a human.
You can try fanfiction.net. It has a vast collection of fan - made stories, including many 'ash team rocket fanfiction' pieces. Just use the search bar and type in relevant keywords to find what you're looking for.
One great 'ash team rocket fanfiction' is 'The Team Rocket's Unlikely Alliance'. It shows an interesting scenario where Ash and Team Rocket end up working together for a common goal. The story is full of humorous exchanges and unexpected plot twists.
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.