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.
The plot could start with Pikachu getting into trouble during a battle. Buneary comes to his rescue. After that, they start spending more time together. As they do, they discover that they have a special connection. The kiss might occur during a beautiful sunset in the Pokémon forest. Their relationship then becomes a source of inspiration for other Pokémon in the area to look beyond species differences in friendship and love.
In the fanfiction, a new Pokémon arrives in their area. This Pokémon is a bit of a troublemaker and causes chaos. Pikachu and Buneary have to figure out a way to calm it down and make it a part of their community. They use their different personalities, Pikachu's bravery and Buneary's kindness, to befriend the new Pokémon.
In a fanfiction, the baby of Pikachu and Buneary could inherit Pikachu's electric powers and Buneary's cute appearance. It might have soft, furry ears like Buneary and the ability to generate small electric shocks. The story could revolve around the challenges and joys of raising such a unique baby in the Pokémon world.
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.
In a fanfiction, Pikachu could use its electric attacks to protect Buneary from danger. The lemon could be a magical item that gives special powers to Pikachu or Buneary. For example, if Pikachu eats a slice of the lemon, it might gain super - charged electricity for a short time. And Buneary could befriend Pikachu and they could go on an adventure together to find more of these magical lemons.
Their friendship could be shown through simple acts like sharing food. Pikachu could offer some of its favorite berries to Buneary. And when one of them is in trouble, the other could come to the rescue immediately. For example, if Buneary gets caught in a net, Pikachu could use its electric attacks to free it.
The kiss could signify the start of a new relationship between them, perhaps a romantic one. It might be a turning point where they realize their feelings for each other are more than just friendship.
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.
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.
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.