Princess Celestia can raise the sun, has strong magic for ruling and protecting Equestria. Princess Luna can raise the moon and also has magic related to the night, like controlling the stars.
Princess Celestia is the ruler of Equestria, responsible for raising the sun. Princess Luna was once envious and turned into Nightmare Moon. Celestia had to banish her to the moon. But later, Luna was redeemed. They both play important roles in maintaining the balance of day and night in Equestria.
There could be a fanfiction where Nightmare Moon teams up with the main ponies in an alternate universe. In this story, they realize that her power can be used for good when she is given a second chance. They work together to fight a greater evil that threatens all of Equestria.
One popular theme is Luna's redemption. Since she had a dark past in the show, many fanfictions explore how she fully overcomes her inner demons and becomes a more beloved princess. Another theme is her relationship with other ponies, like Twilight Sparkle or Celestia.
Many fans like to create their own versions of Equestria in fanfictions. Nightmare Moon is a great character to use in these alternate universes. She can be the catalyst for change, a villain, or even a hero, and this flexibility makes fanfictions about her really interesting.
You can start by looking on fanfiction websites like Fimfiction.net. It has a large collection of My Little Pony related fanfictions, including many about Princess Luna. Another option is Archive of Our Own (AO3), which also has a great selection of fan-created stories.
Nightmare Rarity is an interesting take in the My Little Pony world. In this story, Rarity is often transformed into a more menacing or 'nightmare' version. Usually, it's due to some dark magic or her own inner turmoil getting out of control. This new form of Rarity might have different powers and a more sinister look, which makes for a great plot device in the overall My Little Pony lore.
Luna might be initially cautious around Godzilla. Given her role as a princess in Equestria, she would likely try to assess if Godzilla is a threat. Godzilla, on the other hand, might be confused by the new environment. Eventually, Luna could use her magic to communicate with Godzilla, perhaps on a telepathic level. They would then start to understand each other's motives and find common ground, like protecting their respective homes from danger.
The theme of forgiveness is also quite common. Luna, being a more benevolent character, might be in a position to forgive Sombra for his past wrongdoings. This can lead to some really emotional and character - building moments in the fanfiction. Also, the theme of magic and its consequences is often explored. How their magic affects themselves and the world around them is a big part of many of these stories. For example, Sombra's dark magic might have unforeseen consequences that Luna has to deal with, or vice versa.
Growth and self - discovery also feature frequently. Both Twilight and Luna can grow as characters in fanfictions. Twilight might learn more about leadership from Luna, and Luna could discover new aspects of herself through her interactions with Twilight. This could be related to their understanding of their powers, their place in Equestria, or their relationships with other ponies.