Themes can vary widely. However, the concept of trust is often explored. How does Ash gain Mewtwo's trust? It could be through his actions in battles, his respect for Pokémon, or his ability to see Mewtwo not just as a powerful entity but as a being with feelings. Additionally, the theme of the underdog (Ash) facing the seemingly unbeatable (Mewtwo) is quite prevalent. It makes for an exciting narrative as we see Ash use his wits and heart to achieve the seemingly impossible task of catching Mewtwo.
There are quite a few on fanfiction websites. Some focus on how Ash's kindness and determination lead to him catching Mewtwo. For example, in a certain fanfic, Ash manages to catch Mewtwo during a battle where he shows Mewtwo that there is more to life than power. He shows Mewtwo the beauty of friendship among Pokémon and humans, which ultimately makes Mewtwo willing to be caught by Ash.
One possible scenario in this fanfiction could be that Ash encounters Mewtwo in a new, uncharted region. Mewtwo, being intrigued by Ash's unwavering spirit, decides to test him. Ash passes the tests with the help of his Pikachu and other Pokemon. This leads to Mewtwo seeing Ash as a worthy trainer and allowing itself to be caught. It would be a great twist on the usual Pokemon-catching storylines.
Obviously, Ash is one of the main characters. He is the Pokémon trainer on a quest to catch Mewtwo.
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.
Well, one possible interesting plot could be Ash using his unique bond with his Pokémon to outwit Mewtwo. Maybe he convinces Mewtwo that not all humans are bad through the kindness shown by his Pokémon team.
One common theme is the idea of acceptance. Since their relationship is rather unusual in the Pokemon universe, the fanfictions often focus on how they accept each other for who they are. For example, Ash accepts Mewtwo's power and complex nature, and Mewtwo accepts Ash's simple yet strong-hearted personality.
One common theme is the exploration of trust. When Ash and Mewtwo's relationship is broken by the idea of betrayal, it makes the reader think about how fragile trust can be. Another theme could be redemption. If Mewtwo has been made to betray Ash against its will, the story might focus on how it tries to make amends.
A common theme is the exploration of power. As Ash becomes Mewtwo, he has to deal with a great amount of power that he's not used to. Another theme is identity. He has to figure out who he is now that he's no longer the regular Ash. Also, friendship is often a theme. His friends may have different reactions to his transformation, and he has to navigate those relationships.
One common theme is friendship. Despite their differences, Ash and Mewtwo often form a deep friendship in fanfictions. For example, they might support each other through tough battles or personal crises.
Adventure is also a big theme. They go on various quests together, exploring new regions and facing different Pokemon. Sometimes, the theme of redemption also appears, especially if the fanfiction is set around a 'bad' Mewtwo that is redeemed through its interactions with Ash and Lillie.