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.
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.
Perhaps an ancient evil spirit that was sealed away long ago. The hatching of the legendary starter's egg is a sign that the spirit can break free. It wants to possess the starter to regain its full power and take over the Pokemon world. This would make for a very dark and menacing antagonist, and Ash would have to use all his skills and the power of his friendship to defeat it.
Well, the plot could involve Ash's unique starter being from a far - off region that was thought to be extinct. This starter could hold the key to an ancient mystery in the Pokemon world. As Ash travels with it, they encounter various challenges related to protecting this starter from those who would use its uniqueness for evil purposes. The starter might also have an ability to transform into different forms depending on the situation, like a more powerful form when facing a tough opponent.
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.
One idea could be that Ash's Bulbasaur gets separated from him in a strange, enchanted forest. As it tries to find its way back, it befriends some local Pokémon that are usually shy and elusive. Another plot could be that Bulbasaur discovers a hidden power within itself during a tough battle. Maybe it can suddenly control plants in a much more powerful way than before.
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.
Maybe the fanfiction could focus on a trainer who gets a starter that's considered weak, like a Caterpie. But through hard work and a series of adventures, the Caterpie evolves into a Butterfree with extraordinary powers. The story could be about how they overcome the prejudice against starting with a 'weak' Pokemon and prove themselves in battles and Pokemon contests.
Well, you could have a fanfiction where the starter pokemon are in a competition not just against other pokemon but also against each other for a special prize. The trainers are caught in the middle, trying to balance their loyalty to their pokemon and the desire to win. Another interesting angle is a story where a starter pokemon is misidentified as a different type at first. Say, a Bulbasaur that looks like a Charmander because of a rare genetic mutation. This causes all sorts of confusion and challenges for the trainer. Also, a plot where the starter pokemon are reincarnations of ancient pokemon heroes and the trainer has to unlock their past memories to help them reach their full potential.
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.
One possible plot could be that Ash finds a Riolu as his starter pokemon. Riolu is initially very shy and distrustful of Ash. But as they go on adventures together, Riolu starts to open up. They face challenges like battling against Team Rocket who tries to steal Riolu for its potential power. Ash has to train Riolu not just in battle skills but also in trusting him completely. This journey of building trust and growing stronger together could be a great fanfiction plot.