One popular 'wmmap fanfic' might be those that explore different character backstories. For example, there could be fanfics that focus on the early life of the main character in a more detailed way than the original work.
Fanfiction.net is also a place to look for 'wmmap fanfic'. There are many writers who post their works there. However, the interface might be a bit different from AO3, but it still has a decent amount of content related to this fandom.
Power and magic are also prevalent themes. Given the nature of the characters, their magical abilities play a role in the fanfics. They might use their powers to protect each other or to overcome challenges. Friendship is also a theme, as their relationship often starts as friends before developing into something more.
There's also 'Lucas and Athy: A Different World'. This fanfic takes the characters to a parallel universe where they have to adapt to new rules and circumstances. It's interesting because it shows a different side of their personalities and how they interact in this new setting. The descriptions of the new world are vivid and make the story immersive.
Tornado of Terror would likely be one of the first to try and take on Evil Saitama. She's powerful and confident, but she would soon realize the extent of his power. Mumen Rider, despite being weak, would still stand up to Evil Saitama out of a sense of justice. He would be quickly defeated but his act of courage might inspire other heroes to step up.
The Joker would probably be both delighted and a bit confused. He's used to Batman being his nemesis, but an evil Batman would change the dynamic. He might initially see it as a new and exciting game, but then realize that an evil Batman could be a real threat to his own chaotic existence. The Joker might try to manipulate the situation to his advantage, but it could easily backfire on him.
Batman would be heartbroken. He has trained Robin and sees him as a partner and almost like a son. He would first try to talk to Robin, to find out what made him turn evil. He would hope that he could reason with Robin and bring him back to the good side.
The other Pokemon in the Pokemon Center where Pikachu is usually treated would be very frightened. Nurse Joy would be at a loss as to what to do, as she has never seen Pikachu act this way. Professor Oak would be extremely concerned and would start researching immediately to find out what could have caused this sudden change in Pikachu. Team Rocket, on the other hand, might initially see this as an opportunity to capture Pikachu more easily, but they would soon realize that an evil Pikachu is far more dangerous than they expected and might end up getting in over their heads.
Kakashi would be disappointed. He has trained Sakura and hoped she would grow into a great shinobi with good values. If she turns evil, he would feel like he has failed in some way. He would likely try to talk some sense into her, but if that doesn't work, he would have to take appropriate actions to protect the village from her actions.