How about a crossover with a fantasy world like Middle - earth? Mewtwo could be a mysterious and powerful being that the characters in Middle - earth encounter. It could either help or hinder their quests, depending on how it views their actions. Maybe it would be interested in the magic of Middle - earth and try to learn more about it.
A Mewtwo crossover with a science - fiction universe like Star Trek could be really cool. Mewtwo's psychic powers might be seen as a threat or a wonder in the Star Trek universe. There could be a plot where Mewtwo has to prove itself to the Federation and help them solve a problem using its unique abilities.
A mewtwo fanfic could focus on its relationship with the scientists who created it. Maybe it tries to understand their motives and eventually forgives them. Another idea is a time - travel mewtwo fanfic, where it goes back in time to prevent its own creation but then realizes that it's an important part of the Pokémon universe. And a third interesting concept could be a mewtwo fanfic that's set in a post - apocalyptic Pokémon world and mewtwo has to use its powers to rebuild society.
A possible plot is that Ash, while in Mewtwo's form, is hunted by those who fear Mewtwo's power. He has to use his own knowledge of friendship and teamwork, which is so characteristic of Ash, to convince the people and Pokemon around him that he's not a threat. This could lead to some really cool battles where Ash, in Mewtwo's body, shows his unique fighting style combined with Mewtwo's strength.
One idea could be that Ash and Mewtwo form an unexpected bond. Mewtwo, being so powerful, could initially be distrustful of Ash. But Ash's pure heart and determination slowly break through Mewtwo's defenses. They might go on a journey together to stop a new evil threat that is targeting Pokemon with strong psychic powers. Another plot could involve Mewtwo using its powers to help Ash understand some of the mysteries of the Pokemon world that he has been curious about for a long time.
A possible plot could be that an evil organization creates a special diaper for Mewtwo which is actually a mind - control device. Mewtwo has to struggle against the control while trying to figure out how to remove the diaper. Or perhaps Mewtwo discovers a group of baby Pokemon who are being mistreated because they are always in dirty diapers. Mewtwo uses its powers to clean and care for them, and in the process, learns about the value of kindness and nurturing.
A possible plot could be that Mewtwo, in an attempt to protect Ash from a powerful and mysterious enemy, undergoes a self - imposed TF. This transformation gives Mewtwo the ability to access a hidden power within itself. Ash, on the other hand, has to figure out how to communicate with this new - formed Mewtwo and together they face the common foe. Throughout the story, their relationship evolves as they learn more about each other's capabilities.
One idea could be a story where Mew and Mewtwo team up to save the Pokemon world from a new, powerful and evil artificial Pokemon created by humans. They have to overcome their differences first, as Mewtwo still has some resentment towards humans and Mew is more open - minded.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
One idea could be that Ash discovers a wounded Mewtwo and nurses it back to health. As they spend time together, they form an unbreakable bond that challenges the way both Pokemon and humans interact. Another plot could involve Mewtwo using its powers to help Ash reach a new level of Pokemon training. For example, it could allow Ash to experience telepathic communication with his Pokemon, enhancing their battle strategies.
One idea could be that Mewtwo and Ash are forced to team up to save the Pokemon world from a new, powerful and evil threat. They start off distrusting each other due to their differences, but as they face challenges together, they begin to understand and respect one another.
A possible plot is that Ash is on a journey to a mysterious place that is said to be Mewtwo's lair. Along the way, he meets various Pokémon and trainers who have either had encounters with Mewtwo or know something about it. When he finally reaches Mewtwo, Ash doesn't try to capture it forcefully. Instead, he shows Mewtwo the beauty of the Pokémon world and the relationships between different Pokémon and trainers. This makes Mewtwo intrigued and willing to be captured by Ash.