Themes of betrayal can be common. If Luna was betrayed by someone she trusted, it would be a very sad situation. This could be a pony she thought was her friend or an ally. The resulting sadness could lead to her withdrawing from others and becoming even more melancholy as she tries to deal with the pain of the betrayal.
One key element is Luna's character. Since she's a complex character with a past of being Nightmare Moon, fanfics often explore her redemption and her relationship with the other ponies. Another element is the magical world of Equestria. Fanfics use this setting to create unique stories around Luna, like her connection to the night sky and moon magic.
Another theme is friendship. Even as a baby, Luna is shown forming bonds with other ponies or characters, like how she might interact with her sister Celestia in a more innocent and playful way. And these interactions help shape her future relationships. For example, in some fanfics, her early experiences with friendship teach her important values that she carries into her later life.
A common theme is friendship. Despite their different personalities, they often end up becoming friends in fanfics. Another theme is magic. Since both are associated with magic in the My Little Pony world, their magical abilities are often explored. For example, they might combine their magic to defeat a common enemy.
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.
Redemption is a very common theme. Luna often has to prove herself after her return, making up for her past actions. For example, in many fanfics, she tries to regain the trust of the ponies in Equestria.
One common theme is friendship. Since Spike and Luna are from different parts of Equestrian society, their friendship can be explored in many ways. For example, how they bridge the gap between a young dragon and a princess of the night.
It could be about Luna's feelings of isolation. In the MLP world, Luna was once banished, and a fanfic could explore her lingering sadness from that experience. Maybe she still struggles with being accepted fully by the ponies, and the story delves into her inner turmoil.
Loss is a very common theme. It could be the loss of a friendship, like two ponies who were really close but then something comes between them. Another common theme is disappointment, like when a pony's hopes and dreams don't come true.
A fanfic named 'Luna's Dark Days' is another example. It tells the story of Luna during a particularly difficult time when she feels like she doesn't belong. The plot is engaging as it shows Luna's journey through her sadness and how she slowly starts to heal with the help of her sister and some of the ponies.
One common theme is self - doubt. Scootaloo often doubts her abilities, whether it's her flying skills or her place among her friends. Another theme is loneliness. She sometimes feels left out or different from the other ponies, which leads to her sadness.