Well, in a 'princess celestia x king sombra fanfic', the character development can be really fascinating. Given their different natures, seeing how they change in relation to each other is great. Maybe Celestia tries to redeem Sombra, leading to a journey of self - discovery for both of them. And the setting of their story, whether it's in Celestia's kingdom or Sombra's domain, can add a lot of depth to the plot.
One interesting aspect could be the exploration of their contrasting personalities. Princess Celestia represents light and harmony, while King Sombra is often associated with darkness. Their relationship in the fanfic might involve a struggle between good and evil, or perhaps an unexpected alliance. It could also show how their different powers interact, creating unique storylines.