One possible fanfiction could be about Ash and Serena going on an adventure specifically to find a rare pink Butterfree. Maybe they hear rumors of a special meadow where these pink Butterfree are sighted. Along the way, they face challenges like wild Pokémon and difficult terrains. Ash's determination and Serena's strategic thinking help them get closer to their goal. When they finally find the pink Butterfree, it's a magical moment, and they learn some valuable lessons about friendship and the beauty of nature.
It could be about Ash's Butterfree coming back in some way. Maybe it returns to Ash after a long time apart. It might show their reunion, and how they interact again, like Butterfree showing its affection for Ash once more.
It could be that the pink Butterfree is actually a transformed Pokémon. In the fanfiction, it could be a Pokémon that was experimented on by an evil scientist and turned into this pink Butterfree. Ash and Serena have to figure out how to turn it back to its original form. Along the way, they discover the scientist's lair and have to battle his other creations. This would be an exciting adventure full of mystery and action.
One possible plotline could be that Ash and Serena are on a journey and Butterfree plays a crucial role in guiding them through a difficult forest. Maybe Butterfree's unique abilities, like its powder or its flight skills, help them overcome obstacles like strong winds or wild Pokémon. It could also be that Butterfree has some sort of connection to a special place that Ash and Serena are trying to reach, perhaps a place full of rare Pokémon or a beautiful meadow.
They could also develop their relationship through exploration. If they discover a new and mysterious place, Butterfree can be the one to first scout the area. Ash and Serena then follow based on Butterfree's findings. They can then share their excitement and awe about the new discovery, which would bring them closer together as a team.
In some fanfictions, Ash reunites with a Pokémon he had to release in the past. For instance, he might reunite with Butterfree. The reunion could be filled with nostalgia as they recall their past times together. Also, there are fanfictions where Ash reunites with his father in a very unexpected manner. This reunion often changes the course of his journey and gives him new perspectives on his life as a Pokémon trainer. Fanfictions like these explore different relationships and how reunions can be a turning point in Ash's life.
One idea could be that Ash reunites with his old Pokémon team in a completely new and mysterious region. They have to face a powerful new villain together. Another could be Ash reuniting with an old friend from his journey who has a new and rare Pokémon that they all need to protect.
Obviously, Ash and Pikachu are the main ones. But we could also see Misty and Brock, Ash's old friends from the beginning of his journey. Maybe Gary too, as he has been Ash's rival for a long time.
Friendship is a common theme. Since they are different Pokémon types, their friendship is often explored, like how they overcome their differences. For example, Butterfree's gentle nature and Beedrill's more aggressive one can create an interesting dynamic.