In a fanfic, Ash might have a chance against Arceus. Ash has proven time and again that he can adapt to different situations. He could potentially find a Pokémon's weakness or use some unorthodox strategy. However, Arceus is the creator of the Pokémon world. It has the ability to manipulate reality on a fundamental level. So, on paper, Arceus should win. But fanfics often play with the rules, so Ash could win through some plot - driven miracle.
One interesting plot could be Ash somehow getting a power - up that allows him to communicate with Arceus on an equal level. Maybe he discovers a new type of Pokémon energy that gives him this ability. Another plot could be that Arceus decides to test Ash to see if he's worthy of some great power or responsibility in the Pokémon world.
In a fanfic where Ash betrays and captures Arceus, it could be a very dark and unexpected turn. Maybe Ash has been influenced by some powerful and malevolent force. This would completely change his character archetype from the heroic trainer we know. It could lead to a lot of chaos in the Pokémon world as Arceus is like a god in that universe.
There might be some new characters introduced in the fanfiction. A mysterious Pokemon trainer who has some ancient knowledge about Arceus and can guide Ash. Or perhaps a group of Pokemon researchers who have been studying the relationship between Arceus and the world for a long time. These characters could add more depth to the story, providing different perspectives on the situation between Ash and Arceus, and helping Ash in his fight against the perceived betrayal.
Perhaps Ash has been deceived by a false prophecy or a mysterious figure. This false information could lead him to believe that capturing Arceus is the only way to save the Pokémon world from an impending doom. So, in his misguided attempt, he betrays Arceus.
One possible plotline could be that the Lake Trio discovers a threat to the world that Arceus is either unaware of or is causing (unintentionally perhaps). So they have to battle Arceus to stop this threat. Another could be that a powerful villain manipulates the Lake Trio into believing that Arceus is a danger and they must fight it.
It's unique because it challenges the traditional narrative of Ash's journey. Instead of just aiming for badges and winning Pokemon Leagues, he now has the most powerful Pokemon. This could lead to him having a more global impact on the Pokemon world. Also, the relationship between Ash and Arceus is likely to be very different from his relationships with his other Pokemon. Since Arceus is so powerful and has a god - like status, their bond might be more of a partnership to achieve greater good rather than the typical trainer - Pokemon relationship.
Begin by thinking of a unique angle for your Ash and Arceus fanfic. What if Arceus made Ash a temporary Pokémon god? This could lead to all sorts of interesting situations. Then, focus on the story structure. Have a clear beginning, middle, and end. In the beginning, introduce the situation, like Ash's unexpected meeting with Arceus. In the middle, build up the conflict and challenges. Maybe other trainers are jealous of Ash's new connection with Arceus. At the end, resolve everything in a satisfying way. Also, use vivid descriptions to bring the Pokémon and the world they live in to life. Include details about Arceus' powers and how Ash adapts to them.
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.