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.
Maybe in the fanfic, Ash gets separated from Pikachu early on in his journey. He then has to prove himself all over again. He could meet new trainers and Pokémon that help him on his way. He might also have to deal with the fact that everyone expects him to have Pikachu. This could lead to a story of self - discovery and growth as he tries to make a name for himself without his iconic partner.
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.
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.
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 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.
The plot could be as simple as Ash training an Electrivire in secret. When Gary challenges Ash to a battle, Ash unleashes the Electrivire, which is much more powerful than Gary expected. Pikachu could also be involved by taunting Gary's Pokémon during the battle, adding to the overall scare factor.
One idea could be that the female Pikachu has a special power that only Ash can help her control. For example, she might be able to communicate with a certain type of Pokémon that is usually very difficult to reach. Ash, with his experience and kind heart, guides her in using this power for good, like helping to resolve a conflict between different Pokémon groups in a forest.
The fanfiction could start with Ash and Pikachu finding a magical lemon. When Pikachu touches it, it gets the ability to communicate with all lemon - related Pokemon. They meet a lemon - scented Jigglypuff that tells them about a hidden treasure in a lemon grove. Ash and Pikachu set off to find the treasure. Along the way, they face many challenges, like crossing a river of lemonade and fighting off other Pokemon who also want the treasure. In the end, they find the treasure which is a special lemon that can heal any Pokemon's wounds.
I'm not sure if there is an exact 'ash x pikachu lemon fanfic' available. Fanfics can be quite diverse, but this combination seems rather strange as Pikachu is a Pokémon. Most Pokémon fanfics focus on more common relationships within the Pokémon universe, like trainer - Pokémon partnerships in a non - strange way.
In a fanfic, Ash might tell Gary a spooky story about an Electrivire that went berserk. With Pikachu by his side for added effect, Ash could describe in vivid detail how the Electrivire was uncontrollable and its power was overwhelming. Gary, being a competitive trainer, would be both scared and intrigued, wondering if he could ever handle such a Pokémon. This could play on Gary's insecurities as a trainer and create a great story of fear and rivalry.
Well, in such a fanfic, a cool plot point might be that Ash, as a Pikachu, has to communicate differently. Since he can't talk like a human anymore, he has to find new ways to convey his thoughts and ideas to his friends. Also, there could be a plot where he discovers new powers that normal Pikachu don't have because he was once human. For example, he might be able to strategize better than a regular Pikachu in battles.