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.
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.
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.
One possibility is that Ash's Ekans could be a special one with unique abilities. Maybe it has a hidden power that helps Ash in battles against stronger opponents. For example, it could have a super-effective type of attack that is not common among Ekans, like a special poison - type move that can paralyze opponents for a long time.
In this fanfiction, Ash could start in a mysterious forest and spot a shiny Celebi. Celebi, being a time - traveling Pokemon, could take Ash on a journey through different time periods in the Pokemon world. He might see how the Pokemon world has evolved over time, meet some of his ancestors or future versions of himself, and have to solve various problems in each time period to keep the balance of the Pokemon world.
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.
The plot could involve Ash stumbling upon a legendary Pokemon in a hidden, ancient forest. Maybe he's on a journey and gets lost, and that's when he discovers this powerful and rare creature.
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.
Well, one possibility is that Ash meets the shiny ghost pokemon early on his journey. Maybe it's a Gastly that has some special power. Ash could then try to befriend it, facing challenges from other trainers who also want this rare pokemon. The shiny Gastly might lead Ash to an old, haunted place full of secrets about pokemon evolution or some long - lost pokemon techniques.
One of the Legendary Pokemon could be Giratina. Despite its somewhat dark and otherworldly nature, Giratina might be intrigued by Ash's light - heartedness and his ability to make friends with all kinds of Pokemon. Dialga and Palkia could also be part of the harem. Dialga, with its control over time, might see Ash as an important factor in the future of the Pokemon world. Palkia, in charge of space, could be interested in how Ash navigates different regions and spaces in his journey. And don't forget about Ho - Oh. Ho - Oh is a symbol of hope and might see Ash as a bringer of hope in the Pokemon world, thus joining his harem.
Well, in this case, a fanfiction being 'legendary' might mean that it gives Ash a very special role. Maybe he's the chosen one to unlock some hidden power of the legendary Pokémon. It could also be about the settings. If it's set in a long - lost, mythical land full of secrets and ancient powers related to the legendaries, that would contribute to it being a 'legendary' fanfiction. The plot could involve Ash facing challenges that are on a scale never seen before, like battles against multiple legendary Pokémon at once or trying to prevent a cataclysmic event that only he can stop because of his connection to the legendaries.