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.
Maybe in a parallel universe, the female Greninja is a princess of a secret Greninja tribe. Ash stumbles upon her and her tribe, and they embark on a journey to save both the Pokémon world and the Greninja tribe from an evil force. Along the way, their relationship blossoms.
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 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.
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.
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.
I'm not a big fan of yaoi fanfiction in general, but in the case of Greninja and Ash, it often involves a creative exploration of their relationship in a non - canonical way. Some of these fanfictions might focus on their strong bond and translate it into a more romantic or emotional connection that goes beyond friendship as seen in the original Pokemon series.
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.
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.
They might have an epic battle against a common enemy together. Greninja could show off some new moves it has learned during their separation, and Ash could use his experience from other adventures to lead them to victory.
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.
Well, one possibility could be that Ash has been on a long journey to find and catch Greninja. Maybe Greninja is a super rare and powerful Pokémon that only appears in a very specific and dangerous area. Ash, being the determined trainer he is, overcomes many obstacles like battling other tough Pokémon and traversing difficult terrains. In the end, he uses his skills and the bond with his existing Pokémon to catch Greninja.