Often, a theme is adventure. The presence of Legendary Pokemon in Ash's world in these fanfictions usually leads to grand adventures. They might journey to hidden places or face off against powerful foes together. Also, there can be a theme of self - discovery for Ash. Interacting with Legendary Pokemon can make Ash realize new aspects of his own capabilities and his role in the Pokemon world. For instance, if he teams up with Lugia, he might learn more about his ability to communicate with and understand Pokemon on a deeper level.
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.
He can become more confident in his abilities. Since Legendary Pokemon are so powerful, if Ash can form a bond with one and work together successfully, it boosts his self - assurance. In these fanfictions, he might start to believe more in his own intuition and skills as a trainer. For instance, if he manages to communicate with and help a Legendary like Celebi, he'll see himself as someone capable of handling extraordinary situations.
I'm not sure as I don't typically read such fanfictions. But I would assume some common themes could be the exploration of a unique relationship between Ash and the legendary pokemon, perhaps some kind of power dynamic due to the legendary pokemon's abilities.
I'm not really into that kind of fanfiction. But you can try searching on some fanfiction websites like FanFiction.net or Archive of Our Own. Just be aware that not all 'ash x legendary pokemon' fanfictions are well - written, and also make sure to follow the site's rules.
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.
Articuno, Zapdos, and Moltres are quite common. Their elemental powers and the mystery surrounding them make for great fanfiction material. Ash might encounter them during his travels and have to either help them or face off against them in some way.
One of the Legendary Pokemon could be Giratina. Despite its somewhat dark and otherworldly nature, Giratina might be intrigued by Ash's light - heartedness and his ability to make friends with all kinds of Pokemon. Dialga and Palkia could also be part of the harem. Dialga, with its control over time, might see Ash as an important factor in the future of the Pokemon world. Palkia, in charge of space, could be interested in how Ash navigates different regions and spaces in his journey. And don't forget about Ho - Oh. Ho - Oh is a symbol of hope and might see Ash as a bringer of hope in the Pokemon world, thus joining his harem.
Arceus is quite popular. Given that it's considered the creator of the Pokemon universe in the official lore, it often has a significant role in fanfictions. Mewtwo is also very common. Its powerful psychic abilities and complex backstory make it a favorite for fanfic writers to explore. Another popular one is Lugia. With its majestic appearance and its connection to the sea, it's often a central figure in many stories.