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.
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.
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.
You can start by looking on fanfiction websites like Fimfiction.net. It has a large collection of MLP fanfics, including ones focused on Sad Luna. Just use the search bar and type in relevant keywords like 'Sad Luna' or 'Luna's Sadness' to find the ones you're interested in.
A fanfic titled 'Luna's Redemption in Celestia's Eyes' is quite interesting. It focuses on Luna's journey towards redemption from Celestia's perspective. It shows how Celestia has always believed in Luna, even during her darkest times. The story is filled with emotional moments that really tug at the heartstrings.
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.
One common theme is redemption. Since Luna has had a troubled past with her banishment and all, many fanfics focus on her trying to redeem herself in the eyes of the other ponies. For example, she might be trying to make up for the bad dreams she caused in the past.
Perhaps in the fanfic, Luna has a nightmare about her past as Nightmare Moon. She wakes up crying because she fears that she could lose control again and turn back into that evil form. This would show her internal struggle with her own identity and the fear of repeating her past mistakes.
One possible plotline could be that Luna sacrifices herself to save Equestria from a powerful and ancient evil. She might use all of her magic and strength to defeat the foe, but in the process, it drains her life force completely.
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.