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.
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.
Maybe the chosen one has to solve a mystery that threatens the Pokemon world. The Pokemon use their different types of skills like water - type Pokemon using their ability to explore underwater areas to find clues. Fire - type Pokemon can keep the chosen one warm in cold and dangerous places while they search for answers.
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.
Ash as the Chosen One in Pokemon fanfiction is a really interesting concept. It gives a whole new layer to his character. He could be given a special Pokemon by some mysterious force that is crucial for his destiny. This Pokemon might be one that no one has ever seen before. His friends would also play different roles, perhaps as protectors or guides to help him fulfill his chosen - one duties. And his battles would become even more epic as he has this greater responsibility on his shoulders.
Well, a great plot might be that Ash enters a special tournament where the grand prize is a chance to catch a Pseudo - Legendary Pokemon. He has to battle tough trainers along the way. Also, it could be that a Pseudo - Legendary Pokemon has some sort of unique ability that Ash discovers he needs to help solve a big problem in the Pokemon world. Maybe it can control a powerful energy source, and Ash must earn its trust to use that power for good.
Team Rocket could still be the main antagonists. Even though they usually have comical failures, in this fanfiction, they could discover Ash's connection to the legendary Pokemon and try to use it for their own evil plans. Maybe they want to capture the legendary Pokemon through Ash or steal the power that Ash has because of his chosen - one status.
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.
I'm not sure specifically as there could be many different takes on it. Generally, it might be about Ash being some sort of chosen one in the Pokémon world. Maybe he has a special destiny, like being the only one who can stop an evil force or unlock a powerful Pokémon's true potential.
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.
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.