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.
I don't know who the author of 'ash's choice fanfic' is. There are so many fanfic authors out there and without more information it's impossible to determine.
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.
Maoni's wife was from Daqing, Hei Long Jiang. However, his wife's name was not mentioned in the search results. Therefore, it was impossible to determine who the fishy wife was.
One of his toughest opponents could be Tyson. Tyson was a very strong trainer in the Hoenn League in the original series. His Pokémon were well - trained and had unique battle strategies. Ash would have to really bring his A - game to defeat him.
It could be Alain. Alain is a very strong trainer in the Kalos region. He has a powerful team and his bond with his Charizard is quite strong. If Ash has to win the Kalos League, facing Alain would be a huge challenge as Alain has a lot of experience and his battling skills are top - notch.
A trainer with a rare and powerful Legendary Pokémon could also be a tough opponent for Ash. In the Hoenn region, there are Legendary Pokémon like Groudon or Kyogre. If a trainer has managed to form a bond with one of them, it would be extremely difficult for Ash to defeat them.
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.
It could be a new character created specifically for the fanfiction. A trainer with some super - rare and powerful Pokémon that Ash has never faced before. This trainer might have unique battle tactics that really test Ash's skills.