One possible plot could be Ash being chosen by Arceus to save the world from a new, mysterious threat that even the Legendary Pokemon are powerless against. He has to travel to different regions and form bonds with various Legendary Pokemon along the way, using their combined powers to face the danger.
In this fanfiction, Ash could be reincarnated as a Legendary Pokemon himself. As a Legendary, he has no memory of his previous life as a human. He starts to explore the Pokemon world from a different perspective. He meets other Legendary Pokemon like Lugia and Ho - Oh, who sense something different about him. Eventually, through a series of events and battles, his memories start to come back. He then uses his knowledge of both the human and Pokemon worlds to stop a great catastrophe that is about to befall the Pokemon world.
A great plot might be that Ash accidentally stumbles upon a hidden realm where Legendary Pokemon live in seclusion. His presence disrupts their normal life, but instead of being angry, they are intrigued by this human. Ash shows them his love for Pokemon through his actions. As a result, one by one, the Legendary Pokemon start to follow him around. His harem grows, and with their combined powers and his leadership, they start to take on challenges like saving other Pokemon from a new and dangerous disease that is spreading and is said to be related to an ancient curse that only the Legendaries can help break.
One idea could be Ash encountering a Legendary Pokemon that has lost its memory. Ash then tries to help it regain its memories while protecting it from poachers. Another could be a time - travel plot where Ash is sent back in time and has to team up with a Legendary Pokemon to prevent a great disaster from happening in the past that would change the future of the Pokemon world.
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.
A third plot idea might be that Ash is the chosen one to bring peace between the different factions of legendary Pokemon. There could be some legendary Pokemon that are at odds with each other, and Ash, with his unique bond with Pokemon in general, is the only one who can bridge the gap. He would have to understand the reasons for their conflicts, which could be rooted in ancient history or disputes over territory. Along the way, he'd have to convince both sides that cooperation is better than conflict, and he might use his own experiences with his Pikachu and other Pokemon friends to show them how different species can get along.
In a fanfiction like this, the plot could revolve around the fact that the egg is the key to restoring balance in the Pokemon world. The legendary starter that hatches from it is initially weak but grows rapidly under Ash's care. However, there are forces at work that want to use the starter for their own evil gains. Ash has to team up with his friends and other Pokemon to fend off these threats. The story could also explore the relationship between Ash and the legendary starter as it grows, how they learn from each other, and how their bond becomes unbreakable.
Maybe Ash discovers a hidden power within himself that is linked to the legendary Pokemon. This power could be awakened through a series of dangerous adventures, like facing off against an evil team that is trying to capture all the legendaries. As he progresses, he gradually transforms into a being with some of the abilities of a legendary Pokemon.
If Ash starts with a legendary pokemon such as Rayquaza, it could be that there are abnormal weather patterns in the Pokemon world. Rayquaza has the ability to control the weather. Ash and Rayquaza team up to figure out what is causing these extreme weather changes. Along the way, they meet other trainers who are also affected by the weather problems. Ash has to prove that he is worthy of having a legendary pokemon by using Rayquaza to restore balance to the weather.
In some 'legendary pokemon and ash fanfiction', there might be a plotline where Ash travels to a hidden realm where only legendary pokemon exist. Here, he has to prove himself worthy to the legendary pokemon in order to gain their help in his journey to become a pokemon master. This could involve facing various trials and challenges that test his skills, courage, and friendship with his pokemon.
The plot might involve Ash being hunted by villains who want to use his connection to the legendary Pokemon. His friends in the fanfiction could be either helping him keep his secret or be in danger because of it. Also, his journey to become a Pokemon master could be intertwined with his search for his parent's true identity and purpose, which was perhaps hidden from him for his own safety.
In the fanfiction, perhaps there's a plot where Ash has to save a legendary pokemon from an evil organization. He might use his experience and the trust of his other pokemon to rescue it. This would not only show his courage but also how much he values all pokemon, whether they're common or legendary.