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.
One possibility is that their reunion is bittersweet. Greninja has been through a lot on its own and Ash has also had his own adventures. At first, there might be some hesitation, but then they realize how much they've missed each other. They then decide to team up and enter a high - stakes Pokémon tournament to prove their worth together.
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.
Well, they might have a really emotional moment. Percy could run to Annabeth and just hug her tightly. They would probably share all the things that happened to them while they were apart. Maybe Percy would tell her about some new adventures and the challenges he faced, and Annabeth would listen intently.
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.
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.
Well, perhaps Ash has returned to Kanto for a Pokemon tournament. As he's exploring the old stomping grounds, he stumbles upon Charizard who has been living in the Charicific Valley. At first, Charizard is a bit hesitant, but then it recognizes Ash. They then go on a journey together through Kanto, revisiting old friends like Brock and Misty, and along the way, they have to deal with some rogue Team Rocket members who are up to their old tricks. This reunion rekindles their unbreakable bond.