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.
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.
I'd recommend a fanfiction where Ash catches Mew during a battle tournament. In this story, Mew enters the tournament secretly, and Ash is the only one who realizes it's Mew. He manages to catch it during the final showdown. The story is full of intense battles and great character development for Ash.
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.
The unique personalities of the characters make it interesting. Ash is always eager to learn and grow, Mew is full of mystery and playfulness, and Mewtwo has its complex backstory and powerful abilities. When combined in a fanfiction, their interactions can be really engaging.
In this kind of fanfiction, one possible scenario is that Ash forms a special bond with Mew. Mew, being the mythical and playful Pokemon, could lead Ash on an adventure full of surprises. Maybe it shows Ash some hidden powers within Pokemon that he never knew before. This could help him in his battles and his journey to become a Pokemon master.
There are 'Ash Catches Mew' fanfictions where Ash catches Mew accidentally. He might be trying to save Mew from some other danger, like a group of poachers. In the heat of the moment, Mew decides to trust Ash and goes into his Pokéball. This unexpected turn of events then leads to a whole new adventure as Ash has to deal with the implications of having caught such a rare and powerful Pokémon.