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.
I'm sorry, but most fanfics about a main character like Luna dying can be quite sad and not very common. You could try searching on major fanfic platforms like Fimfiction.net. They might have some stories related to this theme, but it's not a very positive or popular topic in the My Little Pony fanfic community.
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.
Well, a fanfic about Rainbow Dash dying could be a very dark and emotional take. It might explore the reactions of the other characters in the My Little Pony universe. Maybe it shows how her friends like Twilight Sparkle or Fluttershy would deal with the loss. It could also be a way for the writer to explore themes of mortality and the impact one character has on an entire fictional world.
I'm not sure specifically as there could be many different 'My Little Pony' fanfics where Celestia dies. It could be about the reactions of the other ponies in Equestria, like Twilight Sparkle having to step up in a new way without Celestia's guidance. Maybe it also explores how the balance of day and night is affected if Celestia is no longer there to control the sun.
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 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.
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.