Well, it often combines the unique charm of a human character interacting with the magical and regal Luna from MLP. The contrast between their worlds and personalities can create really interesting storylines.
You can try looking on fanfiction websites like Fimfiction or Archive of Our Own. These platforms often have a wide variety of My Little Pony fanfictions, including those with a Human x Luna theme.
One idea could be that a human stumbles into Equestria and meets Luna. Luna is intrigued by this strange being from another world and they go on adventures together, exploring the magic of Equestria. The human could teach Luna about human culture and in return, Luna could show the human the wonders of pony magic.
The relationship can start with curiosity. The human might be curious about Luna's powers and appearance as she is a moon princess. Luna, on the other hand, could be curious about the human's different nature. They can start by simply asking questions about each other's worlds.
One great MLP Luna human fanfic could be 'Luna's Human Adventure'. It explores Luna in a human world, facing new challenges and making friends. It has an interesting plot where Luna has to adapt to human customs and school life.
A common theme is identity. Since Luna is in a human form, she often has to grapple with who she is now compared to her pony self. Another theme is acceptance, both her accepting her new human form and the humans around her accepting this strange new being in their midst.
Another theme is self - discovery. The OC usually has some sort of journey where they learn more about themselves, and Luna also often discovers new aspects of her personality or her role in Equestria through her interactions with the OC. For example, in some fanfictions, Luna realizes she can be more approachable through her time with the OC.
Friendship is a very common theme. Since Twilight and Luna are both important characters in MLP, their friendship is often explored, like how they support each other through different challenges.
One common theme is friendship. Given that they are two characters in the MLP world, their relationship often showcases the power of friendship. Another theme is magic, as both characters are associated with celestial and magical elements in the MLP lore.