One idea could be creating an OC who was originally a regular Trainer but got corrupted by Team Rocket's ideals. For example, they might have lost a big Pokemon battle and felt so defeated that they were easily swayed by Team Rocket's promises of power. Another idea is an OC who is a double agent within Team Rocket, secretly working for the good guys to bring down the organization from the inside. They could have a unique Pokemon partner that helps them in their undercover work.
One idea could be that Ash gets transformed into a Pokémon and has to figure out how to communicate with his friends. Another could be that Team Rocket manages to use a new device to transform Pikachu into a different type of Pokémon. Maybe Ash and Team Rocket end up having to team up when a new, more powerful villain emerges who wants to transform all Pokémon into mindless servants.
One interesting Pokemon Team Rocket fanfiction could be about a Team Rocket member who has a change of heart. For example, they might start to see the beauty in the relationship between trainers and their Pokemon and decide to secretly help the heroes instead of causing trouble.
One interesting storyline could be Team Rocket actually helping Ash in a dire situation. Maybe they realize that a greater evil is about to destroy the Pokemon world and they decide to team up with Ash for once. Their skills in stealing and sneaking around could be useful in gathering information about this new threat.
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.
One idea could be creating an OC who is a long - lost relative of one of the A - Team members. For example, a niece or nephew who has unique skills and joins the team on a special mission. This can add a new dynamic to the group and explore family relationships within the context of their adventures.
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.
There could be a fanfiction where Team Rocket finally manages to steal all of Ash's Pokémon. But instead of using them for evil, they realize the strong bond between Ash and his Pokémon and start to have a change of heart. This would be a great exploration of the themes of redemption and the power of friendship in the context of the Ash vs Team Rocket dynamic.
One idea could be creating an OC that is a hybrid of two different Pokemon types. For example, a half - water, half - electric type with unique abilities. Another idea is to make an OC that has a special bond with a legendary Pokemon, perhaps they are the chosen one to help the legendary in a time of crisis. Also, you could have an OC from a different region with a completely new set of Pokemon not seen before in the main series.
One idea could be a story where a new trainer forms a team with charm Pokemon like Clefairy, Jigglypuff and Sylveon. They enter a beauty contest within the Pokemon world and face various challenges and rivalries along the way.
One idea could be to create an OC that has a unique typing combination not commonly seen, like a Ghost/Grass type. This could lead to interesting storylines in the fanfiction, such as how they interact with other Pokemon and trainers. Another idea is to give the OC a special ability that sets them apart. For example, an ability that allows it to communicate with wild Pokemon more effectively, which could be useful in various fanfiction scenarios.