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.
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.
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.
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.
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.
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.
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.
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 popular Ash and Ash - Greninja fanfiction could be those that focus on their adventures in a new, unexplored region. In these stories, they might face unique and powerful Pokémon, strengthening their bond along the way.