The plot could involve the human son having special powers. Maybe he can communicate with the moon like Princess Luna. He might go on adventures in Equestria, making friends with ponies and learning about their world.
One idea could be that a human stumbles into Equestria and meets Princess Luna. Luna is initially suspicious but as she gets to know the human, they form a bond over their shared love for stargazing. Another could be a human who has the power to enter dreams like Luna, and they team up to fight a nightmare - inducing villain in both the human world and Equestria. Or perhaps a human artist who is inspired by Luna's beauty and grace, and through his art, he catches her attention and a friendship develops.
First, understand the characters well. Research Princess Luna's personality - her past, her relationship with her sister and her subjects. For the human character, define their traits clearly. Then, create an interesting setting. It could be a mix of the human world and Equestria. Also, focus on the relationship development between Luna and the human. Show how they grow to understand and care for each other.
He would probably interact with the ponies in a curious way. Since he's human, he might introduce new concepts to them. For example, he could show them human games or ways of thinking.
Then, plan out the plot. It could be a simple story about the human teaching Luna about human technology, or a more complex adventure where they have to save Equestria together. Don't forget to add details about the setting, like the beautiful landscapes of Equestria or the human's hometown if it's relevant to the story.
They often interact as teacher and student. Luna, with her vast knowledge of the old magic, can teach Twilight things that are not found in modern magic books. For example, Luna might show Twilight how to communicate with the spirits of the night.
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.
Well, in a fanfic, Princess Luna's daughter could be a very interesting character. She might have a special bond with Twilight Sparkle, learning from her about friendship and magic. Her appearance could be a blend of Luna's ethereal beauty and some new elements. Maybe she has a mane that shimmers like the night sky with little twinkling stars in it. And her cutie mark could be something related to the moon and a new concept, like a moon - flower that only blooms at night. She could use her powers to protect Equestria during the night, just like her mother, but also bring a new sense of hope and mystery to the land.