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.
There could be a mysterious and powerful dark - type Pokemon as the antagonist. It senses that Pikachu has the potential to evolve into a very strong electric - type Pokemon and sees it as a threat to its own dominance in a certain area. So it constantly attacks Pikachu and tries to disrupt any process that might lead to its evolution.
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 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.
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.
It could be a trainer who hates Pikachu and wants to capture all of them. This would put Ash, as a Pikachu fan, in direct conflict with this trainer.
A powerful and corrupted Gengar could be a great antagonist. Gengar could use its ghost - type powers to create illusions and traps within the mystery dungeon, making it difficult for Pikachu and its friends to progress. It might be after a powerful artifact hidden in the dungeon and will stop at nothing to get it.
Team Rocket could still be the main antagonists. They might see Ash as a Pikachu and try to capture him more than ever, thinking he's a special Pikachu. Maybe they believe he has some sort of new power because of his transformation.
Well, if Ash's Pikachu evolved into Raichu in fanfiction, it could change the whole dynamic of their journey. Pikachu's evolution might bring new power levels and moves, making battles easier or more complex. Ash would need to adjust to the new Raichu, and their bond might be affected. Maybe Pikachu's cute factor would be replaced with a more intimidating Raichu presence. It could also lead to new storylines where Ash has to deal with the jealousy of other Pokemon or trainers who are envious of his powerful Raichu.
If Pikachu evolves in a fanfiction, it might have an impact on Ash's relationships with other characters. Some trainers might see Ash as a more serious threat now that his Pikachu has evolved. Others might be jealous or try to figure out how he achieved such a transformation.
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 powerful and jealous Pokémon trainer. This trainer might want to get rid of Ash so he can be the best. He manages to brainwash Snivy into betraying Ash and Pikachu. This trainer would be the main antagonist, constantly trying to stop Ash from achieving his goals and causing trouble for him through Snivy's betrayal.