Well, in this fanfiction, Ash as an Eevee could end up in a Pokémon center. The nurse Joy there is confused by this strange Eevee that seems to have an understanding beyond that of a normal Pokémon. Ash tries to find a way to communicate his true identity. He might get into trouble when other trainers try to catch him, thinking he's just a regular wild Eevee. Meanwhile, his Pikachu is on a search for him, and when they finally reunite, Pikachu has to figure out how to turn Ash back to his normal self.
The plot might involve Ash turning into an Eevee and being found by a group of Eevee and their evolutions. They initially view him as an oddity since he has the memories and some of the behaviors of a human. Ash, as an Eevee, could then learn new skills from his new companions and use his knowledge from being a human trainer to help the group in various situations, such as finding food sources or avoiding predators.
A common theme is the exploration of the Pokémon world from a different perspective. As an Eevee, Ash sees things in a new light. He experiences the Pokémon world on a more intimate level, like understanding the language of Pokémon better. There's also the theme of adaptation. He has to quickly adapt to his new physical form, which includes learning to move, run, and use his new Eevee - specific abilities.
Maybe in this fanfiction, Ash's transformation into a Zorua is a result of a scientific experiment gone wrong. As a Zorua, he discovers new abilities and a different perspective on the Pokémon world. He could use his new form to infiltrate a group of Zorua and Zoroark to learn their secrets and perhaps find a way to reverse the transformation. Along the way, he might have to prove himself to the other Zorua and earn their trust.
Well, perhaps in the fanfiction, Ash is exposed to a mysterious Pokémon's special ability that causes the transformation. As a girl, Ash discovers new aspects of himself. He might find that some Pokémon are more attracted to his new form and are more willing to be caught or trained. He could also use this new identity to his advantage in battles, like using the element of surprise against opponents who are used to facing the male Ash.
Well, perhaps it could be about a magical event that causes Ash to transform into a baby. Then his friends have to take care of him while also trying to find a way to reverse the transformation. Maybe Pikachu has to protect baby Ash from other Pokémon who might mistake him for an easy target.
Well, the plot could involve Ash's transformation into Riolu being a result of some magical or scientific experiment gone wrong. Maybe he has to adapt to his new form and learn to use Riolu's abilities like Aura Sphere. He might also struggle with being recognized by his friends at first.
Well, perhaps Ash could be transformed into a Mew through some magical encounter. He might then use his new form to explore the Pokémon world in ways he couldn't before, like flying high above the clouds and seeing the landscapes from a whole new perspective.
One possible plot could be that Ash suddenly transforms during a battle. His Pokémon friends are shocked at first. But then they realize he has gained some new Pokémon - like abilities. For example, he could communicate with Pokémon in a whole new way, understanding their deepest thoughts and feelings. This new form of Ash could lead his Pokémon team to victory in a big tournament, facing off against tough opponents and using his unique abilities to outwit them.
The plot could start with Ash stumbling upon an ancient artifact or a cursed place. As he touches or enters it, he is suddenly transformed into Latios. At first, he struggles to control his new form and powers. He might accidentally cause some chaos in the Pokémon world. His friends then set out to find a way to turn him back. Along the way, they encounter various obstacles and meet other Pokémon who either help or hinder their quest.
Maybe Ash gets magically transformed into a Latios. He then discovers he can fly and uses his new powers to explore regions he couldn't reach before as a human. He might also meet other Pokémon in a different way and form new friendships.
Well, it could start with Ash being transformed into a Pikachu in a mysterious forest. His friends are initially confused but soon realize it's him. As a Pikachu, Ash discovers new abilities he didn't know about before. He might be able to sense things like incoming danger more acutely. He could also have a hard time convincing his other Pokémon that he is still their trainer. Throughout the fanfiction, there could be a series of comical and exciting events as he tries to adapt to his new form and find a way to reverse the transformation.