Maybe Team Rocket could have a new member who is a ninja. This new member could be Ash's main rival. They would try to steal Ash's pokemon in more sneaky and ninja - like ways. And Ash would have to use his ninja skills to protect his pokemon and defeat this new rival from Team Rocket.
It could be a local Kalos trainer who has been training specifically to become the Kalos champion for years. This trainer might have a team of rare and powerful Kalos Pokémon and a very strategic battle style. He or she could see Ash as an outsider and a threat to their long - held dream of being the champion.
Gary could be his main rival. Gary has always been competitive with Ash, and now that Ash is a Frontier Brain, Gary might see it as a new challenge to overcome. He would try to prove that he is better by winning battles in Ash's Frontier facility.
Obviously, Ash is one of the main characters. He is usually at the center of the story, using his skills as both a Pokemon trainer and a ninja. Pikachu is also a very important character, often using its electric powers in combination with Ash's ninja - like strategies. Sometimes, other trainers like Brock and Misty can also be important characters, providing support or joining in on Ash's ninja - themed adventures.
There could be some new rivals as well. Maybe there's a young, up - and - coming trainer who has heard a lot about Ash and wants to make a name for himself by defeating the new Frontier Brain Ash. Also, an old acquaintance from one of his previous travels who has also developed into a strong trainer could emerge as a rival. They might have different views on Pokémon training, which would lead to intense battles.
One possible rival could be a cocky young trainer named Max who thinks he's better than both Ash and Giselle. He always tries to one - up them in Pokemon battles. Another rival could be an experienced trainer named Lily. She has a collection of rare Pokemon and is constantly challenging Ash and Giselle to prove her superiority. There could also be a team of two rivals, Jack and Jill. They work together to try and outwit Ash and Giselle in any Pokemon - related competition.
It could be a new character, someone who has a long - standing grudge against Ash's group. This character might manipulate Ash into betraying his friends by preying on his insecurities or desires.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
Maybe a powerful and evil pokemon trainer who has a vendetta against Ash. Now that Ash is in a vulnerable pokemon form, this trainer could try to capture and control him. The trainer might have some dark motives related to using Ash's unique situation to gain power in the pokemon world.
There could be a new character, a rival to Ash's dad from his younger days when he was an active Pokemon trainer. This rival could reappear in the fanfiction, causing some trouble or adding some drama to the story.
Misty is a very likely candidate. She has been with Ash from the early days of his journey. They have a deep bond and have shared many adventures together. Their relationship has always had a bit of a romantic undertone, so in a fanfiction, it would make perfect sense for them to get married.