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.
A cool plot could be that the Ash - Greninja fusion is mistaken for a new and dangerous Pokémon species. So, they have to prove that they are on the side of good while also trying to find a way to separate when needed. They might enter Pokémon battles to show their good intentions and end up making new friends in the process.
The fusion's character can be developed by highlighting the combination of Ash's determination and Greninja's grace. For example, in battles, it could show a tenacious fighting spirit but also a fluid and elegant style of combat.
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 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 is that Greninja is framed for a crime it didn't commit. It then has to go on the run while trying to prove its innocence. Along the way, it teams up with other outcast Pokémon and together they uncover a conspiracy that goes far deeper than they ever expected.
Well, it could vary a great deal. In some 'ash greninja returns fanfiction', it might focus on Ash and Greninja reuniting after a long separation. Maybe Greninja has been in a far - off place training or on a mission, and now it comes back to Ash to help him in a big battle. They then team up again, with Greninja using its unique Water - Shuriken and other moves to take on powerful opponents.
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, a plot could involve Yoshi getting lost in a strange new world and Greninja being the only one who can guide him back home. Along the way, they face various challenges that test their friendship. For example, they might encounter a powerful storm that Greninja has to use its ninja skills to shelter Yoshi from. And as they journey, they learn more about each other's cultures and become great friends.
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 interesting Ash Greninja fanfiction could be about Ash and Greninja going on a special adventure in a mysterious new region. They might encounter rare Pokémon and have to overcome unique challenges together.
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.