One idea could be that Ash discovers a young and injured Mew. He takes it upon himself to nurse Mew back to health. During this time, they form a deep bond and Mew starts to teach Ash some unique Pokémon moves that are not commonly known. This new knowledge gives Ash an edge in his battles.
Well, it could start with Ash having a strange dream about Mew. When he wakes up, he finds a strange energy signal that leads him to Mew. Mew is in trouble because it has lost its memory. Ash then tries to help Mew regain its memory. As they travel together, Mew slowly remembers bits and pieces of its past, and Ash gets to learn a lot about the history of Pokémon from Mew.
Team Rocket could be the main antagonists. They might hear about Ash's encounter with Mew and try to capture Mew for their own evil plans. They could set up all sorts of traps and schemes to try and separate Ash from Mew and steal Mew away.
One possible plot could be that Ash discovers a Larvitar in the wild. He decides to make it his starter Pokémon. They then embark on a journey together, facing various challenges like battles against other trainers and wild Pokémon. Larvitar evolves over time, growing stronger with Ash's guidance and training.
Well, the plot could be about Ash starting his journey with a Pidgey as his first Pokémon. Maybe he faces challenges in training it to be strong enough to take on gym battles. He could encounter other trainers who underestimate him because Pidgey is a common starter, but Ash proves them wrong as he and Pidgey grow together.
The plot could start with Ash getting Riolu as his starter Pokémon instead of Pikachu. Riolu might be a bit of a troublemaker at first, not fully understanding Ash's commands. But as they journey together, they start to understand each other better.
Perhaps in this fanfiction, Ash discovers that he has a deeper connection to Mew than he ever thought. It could involve Mew choosing Ash as a special trainer to carry out a very important mission in the Pokémon world, such as saving a certain region from a new and powerful threat. This would lead Ash to face many challenges and grow in ways he hasn't before.
The plot could be that Ash's powerful starter helps him quickly become a top trainer in his region. He might breeze through gym battles with ease and attract the attention of powerful trainers and organizations.
One possibility is that Ash discovers a young Rayquaza at the start of his journey. Maybe it's injured and Ash nurses it back to health. This could lead to a strong bond between them, and Rayquaza could become his most powerful Pokémon, helping him in battles against tough opponents like Team Rocket or other powerful trainers.
One plot idea could be that Ash and Datini start on a journey to find a lost magical artifact. Along the way, they encounter various obstacles and have to use their combined skills to overcome them.
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.
I don't know the exact plot as I'm not familiar with this specific fanfic.