It's not appropriate at all. Pokemon has a positive and clean image, and creating such erotic content related to its characters, like gym leaders, would be disrespectful to the brand and its fans. It also violates the moral and ethical standards expected in the fan community.
Obviously Ash is one of the most popular characters. And then there are the gym leaders like Brock, Misty (who also has gym - related aspects in some versions), and other gym leaders from different regions.
One idea could be that Ash uses his unique battle style which he has developed over his journey to create a really challenging gym. He might include Pokémon from all the regions he has visited in his gym battles, making it a diverse and exciting experience for trainers. For example, he could have Pikachu lead the charge as always, but also include some of his less common Pokémon like Heracross or Gliscor.
Well, it could be about Lanora challenging Ash to a gym battle. And during the battle, they start to respect each other's skills. Maybe Lanora realizes Ash's determination and they become friends.
The main characters are Lanora and Ash. Lanora, as a gym leader, has her own set of Pokémon and rules in the gym. Ash, on the other hand, is on his journey to become a Pokémon master. In this fanfiction, their paths cross and they interact, which forms the core of the story.
Well, in some ash gym leader fanfiction, there could be a story where Ash has to overcome personal challenges while facing a gym leader. Maybe he's feeling unconfident, but through his Pokémon's loyalty and his own determination, he manages to pull through. The gym leader might also have their own backstory that ties into the battle, making it more than just a fight for a badge.
One popular pairing is Ash x Misty. Misty is a Gym Leader and she has a long - standing relationship with Ash in the Pokémon series, so it's often explored in fanfiction.
The main theme might be about the growth and experiences of boys in the gym environment, such as their physical training, the friendships they form, or the challenges they face while working out.
For the Sword and Shield story, I'd recommend having Zacian or Zamazenta as one of your Pokémon. They are very powerful legendaries. Along with them, you could have a Rillaboom for its grass - type moves and high attack. Sylveon is also a great addition for its cute looks and strong fairy - type attacks. And don't forget about a Gyarados, which is a classic and very strong Pokémon. Finally, add a Galarian Slowbro for its unique typing and useful moves.
In the 'Final Fantasy' series, Cloud Strife can be considered. He has his iconic Buster Sword and a shield at times. Cloud is a complex character who battles with his own identity while using his sword and shield to fight against various threats in the game's world, from monsters to powerful villains.