One common theme is Ash forming a special bond with a Legendary Pokemon. Maybe he helps the Legendary in a time of need and they become friends. For example, he could save a Mewtwo from some evil organization and then they team up.
One common theme is the growth of Ash's character. Catching a Legendary Pokemon is no easy feat, and it often forces Ash to step out of his comfort zone, face his fears, and become a more mature trainer. For example, in many fanfics, he has to learn new battle strategies and ways to communicate with a Pokemon that is far more powerful and mysterious than his usual team.
One common theme is the friendship between Ash and the legendary pokemon. Often, they form a special bond that helps Ash in his adventures. Another theme could be the power of the legendary pokemon and how Ash has to learn to handle it. For example, in some fanfictions, Ash might have to control the immense power of a legendary like Mewtwo without getting overwhelmed.
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.
Protection is also a big theme. Often, Ash has to protect the female legendary Pokemon from villains who want to capture or harm it. This shows Ash's heroic side and also the vulnerability of the legendary Pokemon. In some stories, the female legendary Pokemon also protects Ash in return, highlighting the mutual care between them.
Common themes in Pokemon Ash TG TF fanfics include adaptation. Since Ash is transformed, he has to adapt to his new abilities and appearance. Also, the exploration of new powers is a theme. His transformed state usually comes with new powers that he needs to figure out how to use. And there's often a theme of acceptance, both from himself and from those around him.
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.
One common theme is Ash's self - less heroism. He often puts himself at risk to help the legendary pokemon in trouble. For example, when it comes to rescuing a legendary from a natural disaster or from villains.
One common theme is their adventures together. Even in a relationship - focused fanfic, they might still be on Pokemon - related quests. Another is the development of their feelings for each other over time, showing how they gradually fall in love.
One popular 'Ash Catches a Legendary Pokemon' fanfic is 'Ash and the Legendary Bond'. In this fanfic, Ash has an unexpected encounter with a Mew while exploring an ancient forest. The story beautifully depicts their growing connection and how Ash overcomes various challenges to gain Mew's trust. It's a great read for fans who love to see Ash interact with powerful and rare Pokemon.
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.