One common theme could be the battle between light and dark. Luna represents the moon and has an association with the night, which can be seen as a time of mystery and sometimes danger. King Sombra is a dark character. So the fanfic may explore this contrast in terms of their powers and personalities.
Another theme might be forgiveness or redemption. If the fanfic takes a different turn, it could be about Luna trying to redeem Sombra, believing that there is good in him despite his past actions. This would be a theme of second chances and the power of kindness.
Power struggle is also likely to be a theme. Both Luna and Sombra have their own powers. Luna has her lunar powers and Sombra has his dark magic. The fanfic could focus on how they use these powers against each other or how they come to an understanding about the proper use of power.
In terms of the plot, it could start with Sombra's return or some sort of threat that he poses. Luna then steps in to deal with it. The setting within the My Little Pony world would be important. It could be set in Canterlot or some other location. The fanfic might also introduce new characters or use existing ones to support the main plotline between Luna and Sombra.
Power is also a common theme. Sombra was known for his power, and fanfics often deal with how he uses or misuses it. Some stories might have him seeking more power, while others could show him learning to control his power in a more positive way. Another theme is the exploration of his backstory. Writers like to fill in the gaps about his past, like where he came from and how he became so powerful in the first place. This helps to flesh out his character more fully in the fanfic world.
Power and corruption is another common theme. King Sombra was power - hungry in the original story, and fanfics expand on this. They might show how his thirst for power not only affects himself but also those around him. It can also explore how power can corrupt even the best of intentions.
One idea could be a story where Luna and Sombra meet in a dream realm. Luna, being the princess of the night, has the power to traverse dreams. Sombra, once a powerful and dark figure, could be lost in this dream realm. Luna tries to understand his past and help him find redemption. It could explore themes of forgiveness and the power of understanding.
One common theme is power struggle. King Sombra is a powerful and often menacing character, and Twilight is also very powerful in her own right. So, their interactions can be centered around who has more power or how they can use their powers against each other.
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.
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.
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.
In 'King Sombra and Twilight Sparkle fanfic', a frequently seen theme is the clash of different magic types. Twilight has her harmonious magic, while Sombra has his dark magic. Their interactions often revolve around how these two magics interact and influence each other. Another theme is the exploration of the gray area between good and evil. Sombra isn't always pure evil in these fanfics, and Twilight isn't always completely good in her approach to dealing with him. There is also the theme of transformation, as Sombra may transform his ways or his form in relation to Twilight's influence.
Often, one key element is the exploration of Sombra's character. It might show his backstory, like what made him the way he was in the first place.