One common theme is power. Both Naruto characters and legendary Pokemon are known for their great power. So, in the fanfictions, there's often a focus on how the combined power of Naruto characters and Pokemon can be used, whether it's for fighting villains or protecting their worlds.
One common theme is the power of the bond between the trainer and the legendary pokemon. It's often shown as a key factor in achieving great things. Another theme is the mystery surrounding the legendaries, like their origin and purpose.
One common theme is friendship. Humans and Legendary Pokemon often form deep bonds of friendship in these stories. Another theme is adventure, as they usually go on exciting quests together.
The theme of friendship often appears. Consider a baby Zapdos in a fanfiction. It might form a friendship with other pokemon or a trainer. This friendship helps it overcome obstacles and learn about the world around it. Also, the theme of mystery is common. Since baby legendary pokemon are rare and powerful, their origin and purpose often hold a sense of mystery in the fanfiction.
Another theme is the power of love. These fanfictions usually show that the love between the legendary Pokemon is so strong that it can overcome various obstacles, like battles against other powerful Pokemon or natural disasters in the Pokemon world. It's like their love gives them an extra boost of power.
One common theme is the forbidden love aspect. Since Legendary Pokemon are often seen as powerful and different from regular Pokemon, a relationship with a human or even another Pokemon can be considered taboo. Another theme is sacrifice. The Legendary Pokemon may have to sacrifice its power or status for the sake of love.
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.
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.
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.
A theme that frequently shows up is the connection between the pregnant legendary pokemon and other characters. For example, a trainer might form a special bond with the pokemon during this time. Also, the idea of new life bringing hope or change to the pokemon world is a common thread. It could be that the birth of the new legendary pokemon is prophesied to bring some sort of positive transformation.
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.