In this fanfiction, perhaps Ash's parent was a legendary Pokemon who fell in love with a human. Ash could be constantly in a struggle between his human side and the side that has the blood of a powerful legendary. His adventures would then not only be about catching Pokemon but also about coming to terms with his own identity and the great power and responsibility that comes with being the son of a legendary Pokemon. He might face discrimination from some in the Pokemon world who view his heritage as an abomination, while others see him as a symbol of hope and a new kind of unity between humans and legendaries.
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.
It could be Mew. Mew is known for its ability to transform and has a very mysterious nature. If Ash was Mew's son, he might inherit some of that mystery and the power to communicate with Pokemon in unique ways.
The appeal lies in seeing Ash in new and exciting situations. With legendaries involved, his journey becomes more epic. It's like taking the underdog story of Ash in the anime and supercharging it.
In Ash-centered legendary Pokemon fanfiction, often Ash has extraordinary adventures. He might form unique bonds with legendary Pokemon that are not seen in the official storylines. For example, he could befriend a Mewtwo and together they face off against an evil organization trying to clone more powerful Pokemon. These fanfictions allow for a lot of creativity, expanding on Ash's character and his relationships in the Pokemon world.
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.
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.
There are also fanfictions where Ash catches Rayquaza. This could be set during a time when the sky is in chaos due to some dark energy. Rayquaza is drawn to Ash because of his pure heart and determination. Ash, with his usual courage, teams up with his pokemon to assist Rayquaza in restoring balance. As a result, Rayquaza allows Ash to catch it.
It might be. Some Pokemon fans are really into fanfictions that focus on Ash and his adventures in the Pokemon world. If it has an interesting take on Ash's character and his interactions with legendary Pokemon, it could gain popularity.
Another possibility is Ash becoming Lugia. Lugia is known as the guardian of the seas. As Lugia, Ash could explore the deep oceans, discover new species of Pokemon, and safeguard the marine ecosystem. He could use his psychic and flying abilities to interact with both water - and flying - type Pokemon, bringing a new era of harmony between different Pokemon types.
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.