Ash is usually very positive and determined. In Jessie's body, he might try to bring some of that positivity to Team Rocket, which would be a big change for them. Jessie, on the other hand, is more devious and cunning. In Ash's body, she might be more aggressive in Pokémon battles, using strategies that are a bit more underhanded at first until she realizes that Ash's way also has its merits.
A plot could involve that during a chaotic encounter in a mysterious forest, Ash and Jessie's bodies are swapped. At first, they both struggle to adapt. Ash, in Jessie's body, has to deal with her two bumbling Team Rocket partners. He tries to keep them from causing too much trouble while also trying to figure out how to get back to his own body. Jessie, in Ash's body, is confused by his Pokémon's training routines. She decides to use her own unorthodox methods which the Pokémon find strange at first, but eventually, they all start to work together in a new and interesting way to deal with the situation.
Adaptation is also a big theme. Ash has to quickly adapt to the new body's abilities, limitations, and sometimes even different personalities. This can range from getting used to a new physical form in a body swap with a Pokémon, to dealing with the different skill sets of another trainer in a human - human body swap. It's all about how he copes and thrives in the new situation.
Another theme is relationship exploration. This body swap situation can bring out different dynamics in their relationship. For example, they might understand each other's feelings better. Also, there could be a lot of humorous situations as they try to navigate the world in each other's bodies. They may struggle with things like how the other behaves in certain situations or how to interact with their Pokémon in a different body.
Another theme is adaptation. Ash has to quickly adapt to the new body's physical abilities, like different levels of strength or agility. This often leads to some really interesting scenarios where he has to figure out how to battle or move around in a completely different way. For example, if he swaps with a smaller trainer, he might have to find new strategies for catching Pokemon. Also, there's often the theme of friendship. His friends might not recognize him at first, and he has to prove that he's still Ash on the inside, which shows the strong bonds of friendship in the Pokemon world.
Perhaps a major plot twist could be that the body swap is not accidental but part of a bigger plan by a mysterious villain. This villain could be using their body swap to gain access to something that only Ash or Iris can reach in their normal bodies. For example, there could be a special Pokémon or a powerful item. And as they try to figure out how to swap back, they also have to deal with this new threat, which adds a whole new layer of complexity to the fanfiction.
Yes, there are quite a few. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Some stories might focus on how Ash and Nurse Joy deal with the swapped bodies, like trying to carry out each other's daily tasks in a humorous way.
One popular place to find 'ash and may body swap fanfiction' could be Archive of Our Own (AO3). There are likely many stories there that explore this interesting concept. You can search for the relevant tags like 'Ash Ketchum', 'May', and 'body swap' to narrow down the results.
One popular 'ash body swap fanfiction' is 'Swapped Worlds' which has an interesting take on the body swap concept involving Ash. It plays with different Pokémon interactions in the swapped state.
One interesting plot could be that Ash, now in Bonnie's body, has to deal with the different social interactions that Bonnie usually has. For example, he might struggle to interact with Bonnie's friends in the way she does. Another plot could be Bonnie, in Ash's body, trying to handle Ash's Pokémon battles. She might use different strategies that surprise everyone. And a third plot could be them trying to find a way to swap back while also learning more about each other's lives.