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.
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.
Their relationship could develop in a more complex way. Maybe Sombra has a hidden past that Luna discovers, which makes her pity him. This could lead to her trying to help him reform. Sombra, on the other hand, might be initially distrustful but then gradually open up to Luna as she shows him kindness. They could end up as friends or at least allies who have a deep understanding of each other.
Well, one possible plot could be that they start as enemies as in the original lore. But then, an external threat forces them to work together. They gradually learn about each other's pasts and find some common ground. Maybe they both had difficult upbringings in different ways. And in the end, they form an unlikely alliance that changes the power dynamics in Equestria.
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 aspect to consider in this fanfiction could be the character development. Twilight is often portrayed as a studious and kind - hearted character. In a story with Sombra, her intelligence might be used to outwit Sombra's more devious plans. For example, if Sombra tries to take over Equestria again, Twilight could use her magic knowledge to counter his dark spells.
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.
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.
A plotline could involve Sombra's spirit lingering after his defeat. Luna, being attuned to the more mysterious aspects of Equestria, detects this. She tries to communicate with his spirit to learn more about his motives. This could lead to a journey of self - discovery for both of them as they explore Sombra's past and Luna's role in understanding the darkness within Equestria.
I also heard about 'Sombra's New Reign'. This fanfic takes a different approach where instead of being completely evil, Sombra tries to rule in a more just way, which causes a whole new set of challenges and adventures for the ponies in Equestria. It's a great read if you like to see different interpretations of the character.
One great mlp king sombra fanfic is 'The Shadow's Redemption'. It delves into the character of King Sombra, exploring his backstory and a possible path to redemption. The story has some really interesting plot twists and well - developed characters.