One idea could be that the OC is a new pony in Equestria who has the ability to sense the lingering darkness of Nightmare Moon within Princess Luna. The OC could befriend Luna and help her fully come to terms with that part of her past, leading to a stronger and more confident Luna.
The OC can be integrated as a helper. For example, if the story is set during the time when Luna is still struggling with the Nightmare Moon part of herself, the OC could be a kind of spirit guide, showing Luna the way to fully embrace her two sides and find balance.
The 'mlp nightmare moon wins' fanfic could explore different character arcs. For example, Twilight Sparkle might have to face a greater challenge in her quest for knowledge and friendship. Other ponies would also be forced to adapt to a world where Nightmare Moon's will prevails, and their values might be put to the test.
I haven't read this specific fanfic in detail. But generally, in fanfics about Nightmare Moon's return in My Little Pony, they often explore themes like redemption, the struggle between good and evil, and the impact on Equestria. It might show how the ponies deal with this new threat and perhaps how Nightmare Moon's character develops during her return.
In MLP Nightmare Moon fanfics, the theme of forgiveness can be quite prominent. The other ponies in Equestria may have to decide whether to forgive Nightmare Moon for her past actions. And there's also the theme of self - discovery, as she discovers new aspects of herself during the course of the story.
Well, there's a fanfic that I really liked called 'Nightmare Moon's New Path'. In this story, she teams up with Twilight Sparkle to face a new threat to Equestria. It's interesting to see how their relationship develops throughout the story. There are also some fanfics that explore her relationship with the other princesses in a more in - depth way.
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.
One common theme is redemption. Many fanfics focus on Luna trying to redeem Nightmare Moon or vice versa. It's about forgiveness and the power of love between sisters.
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.
A plot might revolve around the friendship of the main ponies being tested. With Nightmare Moon in power, they could be separated or forced to betray each other. There could also be a plot where Nightmare Moon discovers a new threat that she needs the ponies' help to face, despite her victory, leading to a strange alliance.