Well, Pikachu could still be a main character. Even if Ash hates everyone, Pikachu might be the one trying to bring back the old Ash. Maybe Team Rocket could also be main characters. They are always around Ash, and his new - found hatred could make their interactions even more interesting. They might be confused by his change or even try to take advantage of it.
Perhaps Ash hates everyone because he has lost too much in his journey. He has seen Pokémon get hurt, friends leave, and goals unachieved. This build - up of disappointments could turn into hatred for all those around him. He might then isolate himself and the fanfiction could be about his journey to either overcome this hatred or succumb to it.
Well, this kind of fanfiction might be an interesting take on the character. It could explore a darker side of Harry Potter that we don't often see in the original series. Maybe the author wants to show how all the pressure and trauma he has endured could lead to a more cynical view of those around him.
Perhaps the fanfiction community could create some guidelines for writing Alola - related fanfictions. These guidelines could ensure that the stories are in line with Ash's character and the spirit of Alola. If the quality of the fanfictions improves, there's a chance that Ash might not hate them as much.
Well, in some RWBY fanfiction, the 'everyone hates Jaune' trope might be used to create drama. Maybe the author wants to explore how Jaune would grow and overcome such extreme dislike from others. It could also be a way to put him in difficult situations for character development.
Perhaps in this fanfiction, Naruto discovers some dark secrets about the village. Secrets that make him feel like he's been used all along. Like the village was only using him as a weapon against their enemies. This realization could make him resent everyone in the village, thus hating everyone.
Well, it could be that Percy has had some really bad experiences in his life that made him bitter towards everyone. Maybe he was betrayed by friends in the past, so now he just can't bring himself to trust or like anyone.
In some fanfictions, Serena might have accidentally released one of Ash's Pokémon. Ash is very attached to his Pokémon, and such an act could make him angry and start to hate her.
He might be initially shocked. Ash has always been surrounded by friends and supporters, so suddenly being betrayed by everyone would be a huge blow. He could be in disbelief for a while.