A great fanfiction idea could be that Greninja has a hidden power that only Ash can unlock. As they travel together, they encounter a series of increasingly difficult battles. In one crucial battle, Ash's belief in Greninja is what finally unlocks this power, leading to an epic victory.
One interesting angle for a fanfiction could be a time - travel aspect. Ash and Greninja are sent back in time to when Greninja was just a small Froakie. Ash has to make sure that all the events that led to their current relationship still happen, while also dealing with the challenges of the past. This could lead to some really creative plot twists and character development.
In a fanfiction, Ash and Greninja might face a powerful, new villain. Greninja, as Ash's starter, could have some unique abilities that are crucial in the battle. For example, Greninja might discover a new form that is only possible because of their deep connection, which was formed from the very start of their adventure together.
Ash could discover a hidden place where Ho - Oh resides. There, he finds a starter that has been abandoned. Ash decides to take it under his wing. Ho - Oh, seeing Ash's kindness, decides to guide Ash on his journey with this starter, giving him special powers or knowledge to help them grow together.
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.
In a fanfic, Ash could be turned into a vampire by an ancient, powerful Pokemon with vampiric abilities. This would completely change his relationship with his other Pokemon. Some might be scared of him, while others would stay by his side. He would then have to learn to control his vampire powers and instincts in order to continue his journey. For example, he'd need to figure out how to be in the sunlight without getting burned as he travels from place to place.
One idea could be that Ash discovers a hidden power within himself that allows him to communicate directly with Legendary Pokemon. This power sets him on a journey to understand the secrets of the Legendary Pokemon world. He might start by having an encounter with Mew, who guides him to different places where he can learn more about his new ability.
Another idea is Ash starting with a Bulbasaur. His journey could focus more on nature and plant - type Pokemon evolution. He might get involved in more environmental - related plots, like helping sick plants in the forest with Bulbasaur's powers.
One possible scenario in this fanfiction could be that Ash's journey takes a completely different turn with Latios as his starter. Latios, being a powerful and intelligent Pokémon, might help Ash quickly gain a reputation in the Pokémon world. They could form a unique bond, different from what Ash has had with his other Pokémon in the original series. Ash could use Latios' speed and psychic abilities to his advantage in battles, making him a formidable trainer right from the start.
One idea could be that Ash and May are in a Pokémon competition held in a special, romantic location. After May wins, Ash impulsively scoops her up bridal - style in celebration. This could lead to a moment of realization between them about their deeper feelings for each other. It could be a really cute start to a fanfiction.
One plot idea could be that the female Greninja has unique powers that only Ash can help her control. As they train together, they develop a deep bond that goes beyond a typical trainer - Pokémon relationship.
One idea could be that Ash and Greninja's fusion has to battle an ancient evil that threatens the Pokémon world. The fusion might have unique powers that are a combination of Ash's strategic thinking and Greninja's combat skills.