Well, it could be a story about their adventures together. Maybe they team up to face a powerful enemy in a tournament. Greninja's speed and combat skills combined with Delphox's powerful fire - based attacks would make them a formidable team.
Perhaps it's a story centered around a mysterious place that only their Pokemon can access. Ash's Greninja and Serena's Delphox have to work together to solve the secrets of this place. They might encounter various obstacles like strange Pokemon and tricky puzzles. Greninja could use its water - based attacks to cool down areas that are too hot because of Delphox's fire, and Delphox could use its psychic abilities to help Greninja anticipate opponents' moves.
Greninja could become more patient. Maybe at the start it's always eager to jump into battle, but through its experiences with Delphox, it learns to wait for the right moment. For Delphox, it might become more empathetic. Since Greninja has a different fighting style, Delphox could learn to understand and adapt to it.
One possible story could be that the baby inherits the speed and agility of Greninja and the magical abilities of Delphox. Maybe it grows up in a world where it has to learn to control both its water - based ninja skills from Greninja and the fire - based magic from Delphox. It could face challenges from other Pokémon who are either jealous or afraid of its unique powers.
A simple fanfiction could be that Greninja and Delphox are forced to be allies when their trainers are kidnapped by an evil organization. They have to put aside their differences and work together to rescue their trainers. Greninja uses its agility to scout the enemy base, while Delphox uses its fire attacks to create distractions.
One great idea for a fanfiction could be a story where Greninja and Delphox team up to face a powerful, mysterious new Pokemon that threatens their region. They have to combine their unique skills, Greninja's speed and water - based attacks with Delphox's fire - magic and psychic abilities.
One possible plot could be that Serena's mom discovers Ash's true potential as a Pokemon trainer and decides to help him in his journey. Maybe she has some old Pokemon training techniques that she imparts to Ash, which helps him win a big tournament.
Another popular trope is the 'mystical connection'. Some fanfictions explore a special, almost magical connection between Delphox and Greninja. This could be related to their elemental powers or some ancient Pokemon lore. It adds an element of mystery and intrigue to the story, making it more engaging for readers.
There could be unfinished business between them. They might have a shared goal that they haven't achieved yet. Greninja's return to Ash in the fanfiction could be to finally complete that goal, whether it's winning a particular Pokemon tournament or saving a certain place from destruction.
They might have an epic battle against a common enemy together. Greninja could show off some new moves it has learned during their separation, and Ash could use his experience from other adventures to lead them to victory.
In this fanfiction, Greninja might come back with new powers. Maybe it has been on a special training journey and returns to Ash to help him in a big battle.
Well, one possibility could be that Ash has been on a long journey to find and catch Greninja. Maybe Greninja is a super rare and powerful Pokémon that only appears in a very specific and dangerous area. Ash, being the determined trainer he is, overcomes many obstacles like battling other tough Pokémon and traversing difficult terrains. In the end, he uses his skills and the bond with his existing Pokémon to catch Greninja.