A fanfiction could focus on Ash Greninja's unique form and abilities. For example, it could tell a story where Ash has to rely on Greninja's special skills to overcome a powerful, mysterious Pokemon in the Alola region. This would show the significance of their partnership in the context of Pokemon Sun and Moon.
A good Greninja fanfiction in the Pokemon Sun and Moon universe often has well - developed characters. Greninja itself should have a distinct personality, not just be a copy of what we see in the games. The author should give it more depth, like its thoughts, feelings, and relationships with other characters.
In some fanfictions, Ash Greninja's moment of saving Ash from a dangerous fall during a battle in the Alola region is really cool. It shows their loyalty and quick reflexes.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a wide selection of Greninja - related fanfictions from Pokemon Sun and Moon. Just use the search bar and type in 'Greninja Pokemon Sun and Moon' and you'll get a bunch of results.
A new Team Rocket - like organization could be the main antagonist. They might be after Greninja because of its unique abilities. They could try to capture Greninja to use its power for their own evil purposes, like taking over the Alola region.
Obviously, Greninja is one of the main characters as the title suggests its return. The trainer who was associated with Greninja before would also be a main character. They are likely to be central to the story's development.
One idea could be that Greninja returns with a new power that it gained during its time away. It could use this power to help Ash in a crucial battle in the Alola region. For example, it might have learned to control water in a more unique way, like creating ice sculptures that can attack opponents.
One interesting plot point could be Greninja's reunion with its trainer. Maybe after a long separation, there are new challenges they face together in the Alola region. Another could be Greninja's unique abilities being put to the test against new and powerful Pokemon in Sun and Moon.
One possible fanfiction could be about Sun and Lillie going on an adventure together in the Alola region. They might discover a new and rare Pokemon that only appears during a certain lunar phase. Sun could use his skills as a trainer to protect Lillie from any potential danger, and in the process, their relationship could grow deeper.
One possible fanfiction could be about Mallow's journey to discover new and rare ingredients for her cooking. Maybe she ventures into a mysterious forest in Alola and meets a Pokemon that has the ability to guide her to a hidden grove filled with unique berries and herbs that she can use to create a special dish for a big island festival.