One interesting plotline could be Ash's discovery of his special connection to Arceus. Maybe he starts having strange dreams or powers that lead him to uncover the truth. It could also involve Ash facing challenges from those who oppose his new - found status as the 'child of Arceus', like some evil organizations that see him as a threat to their plans.
In some fanfictions, a cool plotline is that Ash is initially unaware of his identity as the child of Arceus. As he travels on his normal Pokemon journey, he gradually shows extraordinary powers that he can't explain. His Pokemon also start to behave differently around him. This leads to a mystery - solving adventure where he, with the help of his friends, figures out his true nature and the responsibilities that come with it.
One possible plotline could be Ash gaining unique powers from Arceus. For example, he might be able to communicate with all Pokémon in a more profound way. Another could be that Ash has a special mission given by Arceus to save the Pokémon world from a great threat. And perhaps he has to face challenges that test his new - found relationship with Arceus and the powers it has bestowed upon him.
Team Rocket might be main characters as well. They could be constantly trying to capture Ash or his Pokemon because of his special status. Their bumbling attempts to take advantage of Ash's link to Arceus could add a lot of comical and action - filled moments to the fanfiction. And of course, Arceus itself could be a major character, guiding Ash in some way or interacting with him directly.
A possible plot for a Pokemon Ash and Arceus fanfiction could be that Ash is summoned by Arceus to a sacred place. There, Arceus reveals that Ash has been chosen to help balance the powers of all the different types of Pokemon in the world. This would involve Ash traveling to different regions and facing challenges related to each type of Pokemon. For example, in a region dominated by fire - type Pokemon, he might have to find a way to cool things down without harming the Pokemon. Another plot could be that Ash accidentally angers Arceus, and as a punishment, he has to go on a journey to prove his worth and respect for all Pokemon. He has to complete tasks like helping a group of endangered Pokemon species thrive again. And, a third plot idea is that Arceus gives Ash the ability to transform into different Pokemon forms for a short time to better understand them, and Ash uses this power to solve various problems in the Pokemon world.
One interesting plot point could be how Ash's training methods change under Arceus's influence. Since Arceus is a powerful and divine Pokémon, it might introduce Ash to new and unique ways of training his Pokémon that are different from what he's used to. This could lead to his Pokémon evolving in unexpected ways or developing new and powerful moves.
One possible plotline could be Ash's discovery of his own latent legendary powers. Maybe he starts to exhibit abilities that are unique to legendary Pokemon, like controlling the weather or having an extraordinary bond with all types of Pokemon.
One possible plotline could be that Ash is raised by Arceus to be a sort of Pokémon messiah. He is taught the true essence of Pokémon and is sent on a quest to save the Pokémon world from an impending doom.
Another plot could be that Arceus is in some sort of trouble and Ash is the only one who can save it by capturing it. This could involve a complex story where Ash has to figure out what's wrong with Arceus and how to use his skills to capture it in a way that helps it.
One interesting plotline could be Ash facing a new and extremely powerful legendary Pokemon that challenges his skills as a champion in ways he's never experienced before. This could lead to him having to develop new battle strategies.
One interesting plotline could be Ash becoming a super powerful trainer overnight in the fanfiction. Maybe he discovers a new type of Pokemon power that gives him an edge in battles.
One interesting plotline could be Mallow and Ash going on a special cooking adventure where they combine their skills. Mallow's cooking knowledge and Ash's love for Pokémon could lead them to discover a new Pokémon that is attracted to Mallow's special dishes. They might then have to figure out how to catch it while also dealing with some challenges from Team Rocket who want to steal the Pokémon for themselves.