One possibility is that Ash has to quickly adjust to Raichu's new power levels. Raichu might be a bit more independent compared to Pikachu, which could lead to some initial clashes in their teamwork. But in the end, they could become an even more formidable team, with Raichu's increased strength allowing them to take on tougher opponents in battles.
Another possibility is a rival trainer who has a Pokemon that is weak against Raichu. This trainer might try to prevent Pikachu from evolving to keep their own Pokemon's advantage in battles. They could try to steal Pikachu's food or training resources to slow down its growth and evolution.
One possible plot could be that Pikachu is exposed to a rare form of electric energy in a thunderstorm that's charged with some mysterious power. This could start the evolution process. It might struggle at first, not understanding the changes its body is going through. As it evolves, it could gain new abilities like enhanced speed and a more powerful electric attack. And it might have to face new challenges in battles, as opponents are now more cautious of it.
A group of Team Rocket - like villains could be the main antagonists. They might want to capture Pikachu while it's in the process of evolving because they believe that an evolving Pikachu would have some special power that they could use for their evil plans. They constantly try to disrupt Pikachu's evolution and the bond between Pikachu and its trainer.
One possible plot could be that Pikachu is exposed to a rare type of thunderstone during a powerful thunderstorm. This thunderstone has been charged with an ancient energy. Pikachu absorbs this energy and begins to evolve. But instead of evolving into Raichu right away, it goes through a series of strange and new intermediate forms, each with unique powers. As it evolves, Pikachu has to learn to control these new powers while also facing challenges from other Pokemon who are either jealous or see it as a threat.
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 some fanfictions where Ash's Gible evolves, it might gain new and powerful moves. For example, it could learn Dragon Claw and become a more formidable opponent in battles against other trainers' Pokémon. It might also develop a closer bond with Ash as it evolves, showing more loyalty and determination to win battles for him.
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.
Definitely. The idea of Ash and Pikachu's friendship is very popular in fanfiction. Their relationship is iconic. Fanfictions could tell stories of how they first met and how that initial connection grew into an unbreakable bond. It could also include side characters from the Pokémon world who notice and are inspired by their friendship, like Misty or Brock seeing how Ash and Pikachu always have each other's backs.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.