Both of them could learn about the importance of individuality. Pikachu might realize that being different like Mimikyu is not a bad thing, and Mimikyu could understand that it doesn't have to be exactly like Pikachu to be accepted. They could both grow to appreciate their own unique qualities as well as those of each other.
There could be a competitive aspect. They might compete for the attention of a trainer or for a particular resource in the Pokemon world. For instance, a rare berry that only grows in a certain area. Their interactions during this competition could range from playful taunts to more serious showdowns. However, through this competition, they could also learn to respect each other's abilities.
One possible plot could be that Mimikyu is jealous of Pikachu's popularity. However, instead of becoming enemies, they end up in a situation where they have to rely on each other. For example, they get lost in a strange part of the Pokemon world. Through this experience, Mimikyu realizes that popularity isn't everything and Pikachu learns to see Mimikyu's true value.
Pikachu might become more empathetic. Usually, Pikachu is known for his energy and courage. But in a fanfic with Buneary, he could learn to understand Buneary's more delicate feelings. For instance, if Buneary is sad because she misses her family, Pikachu could learn to comfort her in his own way, which would make him more in - touch with emotions.
For Pikachu, it could develop from being a bit reckless to more cautious. Maybe after getting into trouble due to its impulsive use of Thunderbolt in a wrong situation. And for Buneary, it could become more outgoing. For example, if it was initially shy around other Pokemon but as it spends more time with Pikachu, it starts to open up and make more friends.
The fanfic could be about Mimikyu being jealous of Pikachu's popularity. Since Pikachu is so well - loved, Mimikyu might try to outshine it in some comical ways. For example, Mimikyu could enter a Pokemon contest where Pikachu is also participating and use its unique disguise abilities to create some chaos and unexpected turns during the competition.
Well, it could be about Pikachu befriending Mimikyu. Maybe Pikachu discovers that Mimikyu is not as scary as it looks under that disguise. They could go on adventures together in the Pokemon world, like exploring a new forest or finding a rare Pokemon.
Well, it could be a story of rivalry at first. Pikachu is quite popular among trainers and other Pokemon, while Mimikyu is often overlooked or feared. Mimikyu might envy Pikachu's status and decide to challenge it to battles. But during these battles, they start to understand each other better. Pikachu realizes that Mimikyu is just lonely and wants some attention. So instead of fighting, they end up working together to help other Pokemon in need. For example, they might save a group of Pichu from a landslide by using their combined electric powers.
Friendship is a very common theme. Since Mimikyu is often a bit of an outsider and Pikachu is so well - known, many fanfics focus on how they can become friends despite their differences.
For Pikachu, remember it's energetic and friendly. It often shows its emotions through its electric shocks, like when it's excited or scared. Pikachu loves to play and is very loyal to its trainer. Mimikyu, on the other hand, is a bit more mysterious. It's shy because it's self - conscious about its appearance. It hides under its disguise. Mimikyu can be a bit envious at times but also has a big heart deep down.
One plot twist could be that Mimikyu's disguise is actually a magic spell that was cast on it by an evil Pokemon, and Pikachu has to find a way to break the spell.