One possible storyline could be Sylveon and Pikachu teaming up to rescue a group of baby Pokemon from a powerful and evil trainer. Sylveon could use its ribbons to distract the trainer while Pikachu zaps the cages open.
For Pikachu, you could develop his curious side more. Maybe he's always eager to explore new areas in the fanfiction, which sometimes gets him into trouble. Sylveon could be shown as the more cautious one, using its ribbons to hold Pikachu back when he's about to rush into danger.
One idea could be that Sylveon and Pikachu go on an adventure in a magical forest together. Along the way, they discover a hidden glade filled with rare berries. As they share the berries, their friendship deepens in a really cute way.
Well, Sylveon's ability to calm others with its presence could be used in a story where Pikachu gets overly excited or scared. For example, if they are in a spooky old Pokemon mansion full of Ghost - type Pokemon. Pikachu might be too afraid to use its electric attacks effectively. Sylveon could calm it down, and then Pikachu could use its attacks more accurately. Also, Pikachu's speed with Quick Attack can be used to get Sylveon out of dangerous situations quickly, like when they are being chased by a large group of wild Pokemon. Sylveon can then use its ribbons to either hold off the pursuers or find a hiding place for them both.
They could interact playfully, like chasing each other around in a meadow. Pikachu might zap a small bug Pokémon and Sylveon could then use its ribbons to play with it.
One possible plot could be an adventure where Pikachu and Sylveon have to team up to rescue Ash and Serena who have been trapped by some evil Pokémon. Pikachu could use its electric attacks and Sylveon its fairy - type moves to fight off the captors.
Friendship is a possible theme. As two different Eeveelutions, they could show how different personalities can come together and form a great friendship. They might support each other through tough times and celebrate together during good times.
Well, it could be about their adventures in a magical forest. Maybe they team up to find a rare and powerful Pokemon that has been lost in the forest. They might face various challenges along the way, like other Pokemon trying to stop them or difficult terrains to cross.
Obviously, the main characters are usually Pikachu. But sometimes, there could be other Pokémon that play important supporting roles. For instance, a Pichu could be a sidekick or a younger Pokémon that the Pikachus are protecting.
One interesting plot could be two Pikachus from different regions meeting and learning about each other's unique electric attacks. For example, one Pikachu might know a special move that the other has never seen before. They could then teach each other and become stronger together.