It's possible that Greninja's return is part of a prophecy in the Alola region. The local people might see Greninja as a sign of great change or a hero. So Ash and Greninja have to work together not just to battle but also to fulfill this prophecy, which could involve saving the Alola region's natural balance or something similar.
The fanfiction might explore the emotional bond between Ash and Greninja in more depth. Maybe Greninja had to overcome great difficulties to return to Ash, and this journey of reunion can be full of emotions. Also, the interactions between Greninja and Ash's new Alola Pokémon friends can add a lot of fun and unexpected twists to the story.
The 'alola greninja returns fanfic' might focus on Greninja's journey back to Alola. It could start with Greninja feeling a pull towards Alola, perhaps due to some unfinished business or a connection it still has with the place. Once there, it could encounter various Pokemon and trainers. There might be a story arc about Greninja finding its place again in Alola, dealing with changes that have occurred since it was last there, and forging new relationships while also rekindling old ones.
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.
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.
Well, in Alola, the battle environment is different. Ash Greninja has to adapt to the new battle rules like Z - Moves. It may start by observing local battles and learning from them. It could also adapt to the more friendly and less intense battle atmosphere compared to other regions. This means it might have to change its battle strategy a bit, not always going all - out from the start. And it might interact with different types of Pokemon that are native to Alola, which could lead to new friendships or rivalries, further shaping its adaptation to the Alola environment.
In this fanfiction, Ash might introduce Greninja to his family. His mother could be surprised but also happy to see such a unique Pokémon. Maybe Greninja would show off some of its cool moves in the backyard.
Since it's a fanfic about Greninja returning to Alola, Greninja is surely a main character. Besides, the fanfic might introduce new characters who play important roles. There could be a mysterious trainer who has a connection to Greninja's past in Alola. Also, there may be Pokemon that Greninja teams up with to face challenges in Alola. These could include both common and rare Alola Pokemon, and their interactions with Greninja would be a big part of the story.
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 plot could be that Ash and Greninja start in a different region than the usual ones. They have to adapt to new Pokémon, new trainers, and new challenges. Along the way, they meet other trainers who are also starting with unique Pokémon, and they form a team to compete in a new kind of Pokémon league. Ash and Greninja would have to learn new battle strategies and overcome their differences to succeed.
One popular story might be about Ash Greninja exploring the unique Alola region, facing new Pokemon and challenges. Maybe it discovers new Z - Moves specific to Alola. Another could be its interactions with the local trainers and Pokemon in Alola, learning about the different cultures and battle styles there.