Yes, there might be some. Fanfiction platforms often have a wide variety of crossovers. You can check websites like Archive of Our Own (AO3). Writers love to combine different fictional universes, and Harry Potter and Smaug from 'The Hobbit' are interesting characters to put together in a story.
Well, it depends on how you define 'good'. There could be fanfictions exploring the concept of a pregnant Smaug in different ways. Maybe some are more comical, imagining how the other characters in the Middle - earth universe would react to such a situation. Others could be more serious, delving into the implications it might have on the lore or the world - building.
Well, in male pregnant Smaug fanfiction, one of the main characteristics would be the unique take on Smaug's biology. Since dragons in Tolkien's lore are quite different from humans, the idea of a male Smaug being pregnant challenges the traditional concepts. It might involve creative interpretations of dragon reproduction and how that would work in a male. There could also be elements of magic or some unforeseen circumstances that lead to this situation.
Well, the plot could involve Harry being on a quest and stumbling upon Smaug's lair. Since Harry has his knowledge of magic and Smaug has his dragon - like power and greed for treasure. Harry could be tempted by the treasure as well, but his moral compass as a wizard would make him try to either reason with Smaug or defeat him in a magical duel to protect the local people of Middle - earth. This would create a really interesting dynamic between the two characters.
Often, there are themes related to the protection of the unborn. Since Smaug is a powerful and feared creature, the idea of something precious (the unborn) within him could lead to themes of safeguarding. Also, there might be themes of change, as Smaug's state would surely bring about some sort of transformation in his behavior or the way others perceive him.
One interesting Bilbo and Smaug fanfiction could be about a scenario where Bilbo manages to outwit Smaug not just with words but also by using some long - forgotten magic. Maybe he uses a spell that makes Smaug see illusions of vast treasures in other places, distracting the dragon from the Lonely Mountain for a while.
Well, one way it could work is if Naruto somehow enters the world of Middle - earth where Smaug exists. Maybe through some kind of magical portal. Naruto's ninja skills like his shadow clone jutsu could be used in interesting ways. For example, he could send clones to distract Smaug while he tries to find a weakness in the dragon's defenses. His speed and agility as a ninja would also give him an edge in combat against the huge and somewhat lumbering Smaug.
A frequently seen theme is the idea of redemption. As Smaug was a rather evil dragon, the fanfictions often explore if the dragon child can break free from that negative legacy. There's also the theme of discovery. The dragon child discovers new places, new friends or foes, and new aspects of itself. For instance, it might discover a love for beauty in a world it was taught to only see as something to conquer. This discovery can lead to a change in its character and goals.