A plot could involve a human accidentally stumbling into the hidden realm of Legendary Pokemon. There, they have to prove themselves worthy to the Pokemon and in the process, they develop a unique friendship. The human then uses this new - found relationship to bring peace between humans and Legendary Pokemon who have long been in conflict due to misunderstandings.
Perhaps a human has a special ability that allows them to communicate with Legendary Pokemon. They are then tasked with helping a Legendary Pokemon regain its lost power. Along the way, they face challenges from other Pokemon and humans who don't understand their mission. This journey of discovery and friendship can be really exciting in a fanfiction.
Maybe a story about a human who accidentally stumbles into the hidden realm of Legendary Pokemon. There, they form a friendship with a Legendary that is an outcast among its kind. Together, they try to change the perception of the outcast Legendary in its own community while also finding a way for the human to return home.
Another plot could be that the human has a special ability that allows them to enter the dreams of pokemon. In these dreams, they learn about the pokemon's past, their hopes, and fears. This knowledge helps the human to better understand pokemon in general and they use this understanding to improve the relationship between humans and pokemon in their world. Maybe they even discover a conspiracy that is affecting the pokemon in their sleep, and they have to stop it.
One interesting storyline could be a human with a special bond to a Legendary Pokemon. For example, a trainer who discovers they have an ancient connection to Mewtwo. They might go on a journey to understand this connection and in the process, face challenges from other trainers or organizations who want to use this connection for their own gain.
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.
Mewtwo is also very popular. Given its origin as a genetically - engineered Pokemon, a human could interact with it in many ways. Maybe a human scientist wants to study Mewtwo further, or a young trainer tries to befriend Mewtwo and help it come to terms with its existence and powers.
One idea could be a world where humans and pokemon have to compete in a series of challenges for the right to co - exist in a new territory. Another could be a story where a human with special powers is pitted against a powerful pokemon in a battle of wits and strength. Maybe a pokemon trainer who loses their memory and has to rely on the help of their pokemon to figure out who they are while also facing off against other humans who want to take advantage of their situation.
One idea could be that a regular pokemon gets exposed to a strange, ancient energy that starts its transformation into a legendary. For example, a Pikachu accidentally stumbles upon an old, forgotten shrine in a forest and absorbs some of the mysterious energy there. Another plot could involve a pokemon trainer who has a special bond with their pokemon, and through that bond, the pokemon begins to evolve into a legendary form to save the trainer from a great danger. Maybe a Charizard turns into a Moltres - like form to battle an extremely powerful enemy threatening its trainer's life.
One idea could be a story where a human trainer has a special bond with a Pikachu that allows them to communicate telepathically. Another could be a human getting lost in the Pokemon world and having to team up with various Pokemon to find their way back home. Or perhaps a story about a human who discovers they have the power to transform into a 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.