In this fanfiction, Ash as a Pikachu could discover new abilities. Since he has the knowledge of a human trainer, he might be able to use Pikachu's electric attacks in more creative ways. He could also form a closer bond with other Pikachu in the wild. And he would have to adapt to Pikachu's diet, which is mainly berries and other small food items.
Well, he might start to see the world from a Pikachu's perspective. For example, he would experience the small size and how everything looks so huge. He could also find it difficult to communicate with his human friends in the normal way since he is now a Pikachu. Maybe he would have to rely on Pikachu's ways of expressing emotions like using electrical signals.
He could find that his relationship with his Pokemon changes. Some Pokemon might be more protective of him now that he's in a more vulnerable form. Others might see him as less of a trainer and more of an equal. Ash would also have to deal with how the Pokemon world views hedgehogs. Are they seen as pests or something more interesting? This could lead to different reactions from different Pokemon species. And as he travels, he might discover new places that are more suitable for a hedgehog - like small burrows or areas with lots of bushes where he can hide and rest.
The way Ash adapts to Pikachu's instincts could be really interesting. He might find himself being more drawn to shiny objects or having an urge to store food in a certain way. And his relationship with his own Pikachu (if it still exists in the story) would be very different. They could become like equal partners instead of trainer - Pokémon.
One possible plot could be that Ash is turned into a Pikachu by a strange spell cast by a mischievous Pokémon. As a Pikachu, he has to figure out how to communicate with his friends. His Pokémon pals at first don't recognize him, but Pikachu - Ash starts using his old battle strategies in a new way. For example, he tries to direct Pikachu's electric attacks in the same patterns he used to direct his other Pokémon's moves in battles. Eventually, his friends realize it's him and they set out to find a way to reverse the spell.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.
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.
In some fanfictions, Pikachu could inherit Ash's dream. Ash had a big dream of becoming a Pokemon Master, and Pikachu might take up that mantle in a way. It could start traveling to different regions, challenging strong trainers, all while carrying the memory of Ash with it. This would show how much Ash influenced Pikachu during their time together.
In this fanfiction, Ash might take really good care of the Pokemon egg. He would probably keep it warm and safe, waiting eagerly for it to hatch. Maybe he'd even talk to the egg, like he does with his other Pokemon friends.
One possibility is that Pikachu's discovery of Ash's Pokemon instincts leads to a new training method. Since Pikachu realizes Ash has this special understanding, they could work together to train the other Pokemon more effectively.
In the 'Ash becomes a Pikachu fanfiction', Ash could have dreams where he is actually a Pikachu. This could lead to some really interesting adventures. For example, he wakes up in a dream as a Pikachu and has to figure out how to use his new form to battle. He might also make new Pikachu friends in his dream world and learn their unique ways of living. This could then have an impact on how he treats Pikachu in the real world when he wakes up, like being even more protective and affectionate towards his beloved Pikachu.
Well, Ash would be devastated. Pikachu has been his closest companion throughout his Pokémon journey. He might go on a desperate search for Pikachu, traveling to all the places they've been together. This could lead to Ash encountering new Pokémon and trainers along the way.