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.
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.
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.
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.
There's a fanfic where Ash catches Mew and Mewtwo during a special tournament. The story focuses on how Ash has to face off against powerful trainers who also have their eyes on Mew and Mewtwo. It's a great read as it shows Ash's growth as a trainer and how he strategizes to keep these two Pokémon safe from those who might misuse their power.
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.
Well, in this fanfiction, Naruto might initially approach Mewtwo with his usual enthusiasm and friendly nature. Since Mewtwo is a very powerful Pokémon, Naruto could see it as a new kind of challenge. He could try to form an alliance with Mewtwo, thinking that together they could achieve great things. Naruto might offer Mewtwo his friendship and loyalty, which could be a new concept for Mewtwo as it has often been a loner.
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.