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.
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.
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.
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 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.
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.
In a fanfiction about Pikachu evolving, it might happen because of a strong emotional bond. Suppose Pikachu is trying to protect Ash from a great danger. In that moment of extreme self - sacrifice and love for Ash, it evolves. This evolution could lead to a whole new dynamic in their relationship. Ash would have to learn how to train a Raichu instead of a Pikachu, and Raichu might be more independent but still deeply loyal to Ash.
It could be a rogue Pokémon trainer who wants to capture Ash - now as Pikachu - for his own collection. This trainer might have a cruel streak and not care about the well - being of Pokémon.
In the 'Ash gets a female Pikachu as a starter fanfiction', one possibility is that the female Pikachu could form special bonds with other Pokémon. Since female Pokémon often have different social behaviors, she could befriend different types of Pokémon more easily. This would open up new storylines for Ash, like being able to access areas or get help from Pokémon that he otherwise wouldn't have. Also, she might be more attracted to certain types of Pokéballs or items, which could also add interesting twists to the story.
They could start by sharing their favorite things, like Ash sharing his Pokemon training experiences and the female Pikachu sharing her favorite places to play in the wild.
One possible reason could be that Ash senses a dangerous situation where Pikachu's full power might attract unwanted attention or put them in more danger. For example, if there are powerful and unseen enemies nearby, Ash might hold Pikachu back to keep a low profile.