It could lead to some really exciting storylines. Maybe Ash and Greninja would face even tougher challenges together and become an unbeatable team.
If Greninja stuck with Ash, their teamwork could evolve to a whole new level. They might face off against stronger opponents and achieve greater victories together. Also, it could deepen the emotional connection between Ash and Greninja, making for more heartwarming moments.
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, perhaps Greninja will encounter some new and strange Pokémon in Alola that it has never seen before. Ash could use this opportunity to train Greninja further, and they might participate in some Alola - specific Pokémon contests or battles together. Greninja's presence could also stir up some friendly rivalries among the local Pokémon and trainers, leading to exciting adventures and growth for both Ash and Greninja.
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.
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.
In fanfiction, Ash and Greninja might have all kinds of adventures in Alola. They could encounter new Pokémon species that are unique to Alola. Greninja could show off its skills in the new environment, impressing the local trainers. Maybe they'll participate in the Alola Pokémon League together and face strong opponents.
One possible scenario is that Greninja is tricked into believing that Ash is endangering its kind. So it decides to turn on Ash. Haruka, on the other hand, having known Ash for a long time, might try to convince Greninja of Ash's true nature. However, Greninja's loyalty is so shaken that it doesn't listen easily. In the end, Ash has to prove his worth and friendship to Greninja all over again.
Well, if Gohan kept training, he'd likely become way stronger and could handle even tougher opponents.
Well, they could have an epic battle side by side again. Maybe they face a powerful new enemy that only their combined skills can defeat. Ash could show how much he's grown since they last parted, and Greninja might have some new moves or abilities too.