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 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.
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.
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.
The 'ash meets pikachu fanfic' often creates new scenarios for their adventures. For example, they might end up in a different dimension where the rules of Pokémon battles are completely different. This allows for a fresh take on their relationship and how they adapt to new situations.
Friendship is a very common theme. Pikachu and Ash have a special bond that is often explored in these fanfics. Their loyalty to each other and how they support one another through different situations.
Brock could be a great addition. He could provide some comic relief and also be the voice of reason. For example, when Ash and Pikachu get into some crazy situations, Brock could be the one to offer practical solutions.
You could give her a distinct physical feature. For instance, instead of the normal brown stripes on a Pikachu, she could have blue ones. This would immediately set her apart. Also, her personality could be more shy and reserved compared to the typical Pikachu, making her reactions to situations different.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.