One possible plot in this fanfic could be that Ash and his Mega Evolved Pikachu have to face a powerful new villain. This villain has a team of strong Pokémon that are causing chaos in the Pokémon world. Pikachu's Mega Evolution gives them an edge in battles. Along the way, they also discover new secrets about Mega Evolution and Pikachu's origin, which further deepens their bond and makes them an even more formidable team.
The power of its electric attacks would be magnified. It could release bolts of lightning that are not only more powerful but also have a wider range. Maybe it could even control the direction of the lightning more precisely, hitting multiple targets at once.
Well, if Ash evolved Pikachu in fanfiction, it might change the way their relationship is perceived. Maybe Pikachu would become more independent or they'd face new challenges because of the evolution.
In a fanfic about Ash and a female Pikachu, the story could be set in a new region. The female Pikachu could be a rare color variant, like pink or purple. Ash would be intrigued by her from the start. They could start training together, and the female Pikachu could be very intelligent and learn new moves quickly. Their journey might include trying to protect the female Pikachu from trainers who want to catch her because of her rarity, and in the process, they grow closer and become an unbeatable team.
One possible plot could be that Ash starts his journey with a different starter Pokémon, like Bulbasaur. Without Pikachu by his side, he might face different challenges and form different relationships. He could struggle more in the early battles without Pikachu's electric power, but then develop unique strategies with his new Pokémon team.
One possible plotline could be that Ash and his Mega Charizard embark on a journey to a new, uncharted region. There, they face a powerful new villain with a team of never - before - seen Pokémon. Along the way, Ash has to train Charizard even further to unlock new Mega Evolutions. They meet new friends who help them in their battles and exploration.
In this fanfic, it could be that Ash is turned into Pikachu as a result of an experiment gone wrong. As Pikachu, he discovers new abilities and senses that he didn't know about before. He might form closer bonds with other Pikachu in the wild and learn their ways of living. He could also have to face challenges like being hunted by other Pokémon that see him as an easy target, and he has to use his new Pikachu - like skills to survive and try to find a way to turn back into his human form.
One possible plot could be that Ash turns into Pikachu and has to learn how to communicate with his human friends in this new form. He might struggle at first, but then discovers that he can use his Pikachu abilities in unique ways to help the group in battles and in solving problems around them.
Another idea could be a more emotional story. The female Pikachu gets separated from her group for some reason. Ash finds her and takes it upon himself to reunite her with her family. During this time, they have to endure harsh weather conditions and dangerous terrains. As they spend time together, they develop a deep connection that is based on mutual care and the shared goal of getting the female Pikachu back to her loved ones.
It could be about Ash and Pikachu meeting a young trainer who is a big fan of them. Maybe the young fan gets lost in the forest and Ash and Pikachu help the fan find their way back. Along the way, they teach the fan some important lessons about Pokémon training, like the importance of kindness towards Pokémon.
I'm not sure if there is a specific 'ash no pikachu fanfic' out there. There are countless fanfics in the Pokémon fandom, but this particular one might be hard to find. You could try searching on dedicated fanfic websites like FanFiction.net or Archive of Our Own and use the relevant keywords to see if it exists.