In certain fanfic ideas, Arceus could be influenced by an evil force. This evil force manipulates Arceus into believing that Ash will cause great harm in the future. So, Arceus betrays Ash as a preventive measure, according to the fanfiction plot. However, this is just a fictional concept and not part of the official Pokémon storyline.
One possible plot could be that Ash discovers a threat to the Pokémon world that only Arceus can stop. But Arceus is somehow corrupted or misinformed. So Ash has to fight Arceus to snap it out of it and save the world.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.
I'm not sure exactly how many fanfictions exist where Ash is the son of Arceus, but it's a very interesting concept. It could lead to all kinds of plot twists. For example, Ash might have some hidden powers or a special connection to the Pokémon world that others don't. This kind of fanfiction can explore the relationship between Ash and Arceus in ways that the original series doesn't.
In 'ash raised by arceus fanfiction', one could expect a lot of creative elements. For example, Arceus being a god - like Pokémon might raise Ash in a way that completely changes his understanding of the Pokémon world. It could involve Ash learning secrets about Pokémon that no one else knows, and this might lead to new adventures and challenges for him.
Well, the plot could involve Ash getting transported to a special realm where Arceus resides. Maybe Arceus sees something special in Ash and decides to test him. Ash could be given tasks that are related to saving the Pokemon world from a great danger that only he, with the help of Arceus, can prevent.
A potential plot could be centered around Ash's role in a prophecy. Since he is the son of Arceus, he is the key to saving the Pokémon world from a great danger. He has to travel to different regions, gather powerful Pokémon, and learn ancient secrets about his father. His friends might initially be skeptical of his new identity but eventually come to support him as he faces numerous challenges and battles against powerful foes.
His rivals could be the ones to betray him. For example, Gary Oak might see Ash's godlike powers as a threat to his own status and decide to plot against him. Gary could spread false rumors about Ash to turn others against him.
It could be Gary. Gary has always been a rival to Ash. Maybe in this fanfiction, Gary gets jealous of Ash's progress and decides to betray him by making false accusations to the Pokemon League about Ash using illegal training methods.
Maybe Team Rocket. They've been after Ash for a long time. If Ash comes back as a champion, they might see it as a threat and betray him in some way. They could pretend to be his friends at first and then stab him in the back to get what they want, like his powerful Pokémon or his championship title.
A fellow trainer in Alola could betray Ash. This trainer might be envious of Ash's success as a champion. He could secretly work with Team Skull or some other antagonists to undermine Ash. For instance, he might give false information to Ash about a battle location, leading Ash into a trap.