One possible plot could be that Ash gets exposed to some strange energy during an adventure. This energy starts to transform him gradually into Mewtwo. As he changes, he begins to experience Mewtwo's memories and powers. He might struggle with this new identity, trying to figure out how to control the immense psychic abilities. Meanwhile, his friends are both shocked and worried, not sure how to deal with this new Ash - Mewtwo hybrid.
One possible plot could be that Ash discovers a female Mewtwo in a hidden location. She's scared and alone, and Ash decides to help her. They form a bond and go on adventures together, facing various Pokémon trainers and challenges along the way.
The plot could involve Ash hearing rumors about Mewtwo's location. He sets out on a journey to find it. Once he does, he tries to communicate with Mewtwo. Mewtwo initially resists but Ash's pure intentions start to sway it.
One possible motive could be that Mewtwo sees Ash as a threat to its own power or freedom. Ash is constantly on a journey to catch and train Pokémon, and Mewtwo, being a powerful and intelligent Pokémon, might view Ash's actions as a potential danger to its kind or its own existence.
Perhaps Mew and Mewtwo are in a fight against an evil force that threatens the Pokémon world. Ash arrives at the scene and manages to communicate with them. He offers his help and through their combined efforts, they defeat the evil. As a result, Mew and Mewtwo see Ash as a true hero and allow him to catch them, leading to a new chapter in Ash's Pokémon journey.
Perhaps the plot involves Ash training with Mewtwo and its clones. He could be learning about their unique powers and abilities, and in turn, teaching them about the values of friendship and cooperation. This could lead to them all becoming a powerful team that faces various challenges together.
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.