One interesting plot could be where the starter pokemon, say a Charmander that is a legendary type in this fanfiction, has to go on a quest to save its kind from an evil force that wants to use their power. It might face many challenges and make new friends along the way.
A great plot can be that the legendary starter pokemon, perhaps a Bulbasaur with unique legendary abilities, gets separated from its trainer in a strange new region. It has to survive on its own, learn about the local pokemon and their cultures, and at the same time, try to find its way back to its trainer. During this time, it uncovers a mystery about the region that is tied to its own legendary nature.
One interesting storyline could be Ash getting a legendary starter that has a unique bond with him. For example, it might be able to communicate telepathically with Ash. This could lead to them having a deeper understanding of each other and facing challenges in a new way.
One interesting plot could be an Eevee starting its journey in a post - apocalyptic Pokemon world. It has to quickly evolve to survive against the new, more dangerous Pokemon species that have emerged. Another plot might involve an Eevee being the chosen one among a group of starters to save a magical forest that is the source of all Eevee evolution stones. And a third could be an Eevee starting in a world where humans and Pokemon are at war, and it has to use its charm and unique abilities to broker peace.
One interesting plot could be a young trainer who gets a starter that is considered weak, like a Caterpie, but through hard work and unique training methods, it evolves into a powerful Butterfree that surprises everyone. Another could be a trainer who has an unexpected bond with their starter right from the start, for example, a Pikachu that refuses to go into a Pokeball and they form a really close relationship like Ash and his Pikachu in the anime.
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.
Often, the antagonists in these fanfictions are popular. Imagine an evil team that wants to capture the legendary starter for their own dark purposes. Their leaders, with their devious plans and menacing auras, can draw a lot of attention. Another popular character could be an old sage - like figure who has knowledge about the legendary starters and can offer guidance to the main characters.
One interesting plot could be Ash forming an unbreakable bond with a Legendary Pokemon. Maybe he helps the Legendary in a time of crisis, like saving it from an evil organization. This would lead to the Legendary choosing to stay by Ash's side and help him in his battles and adventures.
One popular Pokemon Legendary Starter fanfic is 'The Legendary Beginnings'. It has an interesting plot where the main character starts their journey with a legendary starter in a unique region. Another great one could be 'Starter's Destiny'. It focuses on the bond between the trainer and the legendary starter as they face various challenges. And 'Legendary Starter Ascent' is also worth checking out, which has a more complex storyline involving the evolution and power - up of the legendary starter.
One interesting plot could be a trainer who accidentally stumbles upon a hidden realm where legendary Pokemon reside and they all develop a special bond with him, forming a harem. Another could be a story where a young researcher is chosen by the legendary Pokemon to be part of their world and ends up in a harem - like situation due to their unique connection.
Obviously, Ash is one of the main characters. He is the trainer who gets the legendary starter. And of course, the legendary starter itself is also a main character. It has its own personality and abilities that play a big role in the story.
One common plot is that Ash discovers a hidden egg of a legendary starter Pokémon. He then has to protect it from poachers or other trainers who want to use its power for evil. Another plot is when Ash is chosen by a legendary starter Pokémon to be its partner to save the Pokémon world from a great threat.