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.
In this 'harry hates himself fanfiction', a plot could involve Harry being framed for a crime he didn't commit. Everyone at Hogwarts turns against him, and he starts to believe that he must be a bad person if so many people think he's guilty. So he begins to hate himself. Also, it could be related to his relationship with Voldemort. If he starts to see similarities between himself and Voldemort in terms of power or ambition, that could drive him to self - hatred. Another idea is that he fails an important mission and blames himself so much that it turns into self - hatred.
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.
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.
Maybe Harry has had some really bad experiences that made him bitter towards everyone. For example, he could have been betrayed by his friends multiple times.
One possible plot could be that everyone believes false rumors about Harry being a dark wizard. His friends from Hogwarts, including Ron and Hermione, turn their backs on him. Harry then has to find a way to prove his innocence all alone, perhaps by going into hiding and secretly investigating who spread the lies.
One possible plot could be that Ash is kidnapped by a rival trainer group who want to steal his Pokémon. His friends then have to embark on a rescue mission, facing various challenges and battles along the way.
One possible plot could be that Ash discovers Anabel has betrayed him by siding with a rival trainer in a crucial tournament. This could lead to a rift in their friendship and Ash having to find a way to overcome this setback and prove himself without her support.
One possible plot could be that Ash is falsely accused of a serious crime within the Pokémon world, and Greninja, due to some misinformation or outside influence, turns against him. Maybe a powerful villain manipulates Greninja's mind, making it believe that Ash is a threat to the Pokémon world. This could lead to intense battles between them, with Ash being heartbroken and trying to prove his innocence while also trying to save Greninja from the villain's control.
Perhaps Ash is wrongly accused of something in Skyla's hometown. Skyla believes the false accusations and turns her back on Ash, which makes Ash feel extremely betrayed. He then has to prove his innocence not only to Skyla but also to the whole town.
Maybe Ash has some misunderstandings about Dawn. For example, he might think Dawn is too competitive and it makes him uncomfortable.