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 is adventure. Since the original story has a lot of adventure elements, fanfics often continue this. Yona and Hak go on quests together, facing enemies and exploring new lands, which strengthens their bond.
Obviously, the princess would be one of the main characters. There might also be a charming prince or a dashing knight as the male lead. And there could be a scheming villain, perhaps an envious noble or a power - hungry courtier.
One interesting aspect could be its setting. It might be set in a grand, fictional kingdom which adds a touch of magic and allure. The characters in 'la princesa' are likely to be complex and well - developed, with the princess herself having a unique personality.
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.
One popular 'Yona of the Dawn Yona x Hak fanfic' is 'The Journey Ahead' which beautifully explores their relationship as they face various challenges together. It has great character development and a lot of sweet moments between Yona and Hak.