Buneary could also develop leadership skills. Suppose they are in a group of Pokémon trying to achieve a goal. Buneary, with her intelligence and Pikachu's support, could step up and lead the group. This would be a big change from her initial self, and it would show how their relationship can help her grow as a Pokémon.
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.
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.
One common theme is friendship. Since Pikachu and Buneary are both Pokemon, they can form a strong bond in the fanfics, much like how Pokemon form bonds with their trainers in the actual games and anime. They might help each other out in difficult situations.
I know that Archive of Our Own (AO3) also has a wide variety of fanfics. It has a great tagging system. So you can search for Pikachu and Buneary tags, and then sort through the results to find ones that seem appealing to you. Some of the fanfics might have unique storylines, like Pikachu and Buneary going on an adventure together in a different region of the Pokemon world.
A common theme is friendship. Pikachu and Buneary are often shown as good friends, relying on each other. Another theme could be adventure, as they explore the Pokemon world together.
One common theme is friendship. Since Pikachu and Buneary are two different Pokémon, their friendship is often explored in fanfiction, like how they learn to understand each other's differences.
A really interesting plot could be that Buneary gets lost in a strange part of the Pokémon world. Pikachu, being the brave Pokémon he is, goes to find her. Along the way, they encounter various Pokémon with different abilities and personalities. This journey not only tests their physical strength but also their mental strength. In the end, Pikachu finds Buneary, and they return home, with their bond stronger than ever.
Joey could start as a timid trainer and become more confident as he bonds with Rattata. Rattata, on the other hand, might start off as a wild and skittish Pokémon but become more loyal to Joey over time.
Karai could start off as a cold and calculating character, but as she spends time with April, she might become more empathetic. For example, seeing April's kindness towards others could make Karai question her own actions.
Alucard could start to see Anderson not just as an enemy but as a worthy adversary with a sense of honor. This might make him less bloodthirsty towards Anderson specifically. For Anderson, he might come to understand that Alucard is not just a mindless monster but has his own reasons and a complex past.