Magic and the supernatural are often themes as well. Stolas has powerful magic abilities, and fanfics play with how he uses them, discovers new powers, or how his magic affects his relationships. Some fanfics also explore the origin of his magic and how it ties to his family's history.
There's 'Stolas and His Daughter'. This fanfic focuses on Stolas' relationship with his daughter. It shows his more tender side and how he tries to be a good father in the complex world he lives in. It's a heartwarming read.
Another theme is the power dynamic between them. Stolas is a powerful demon with a high social standing, while Blitz is more of an underdog in a way. Fanfictions may play with this power dynamic, sometimes having Blitz gain power or influence in the relationship, or Stolas learning to respect Blitz's agency more. Also, there are often themes of redemption and self - discovery for both characters within the context of their relationship in these fanfictions. They might grow and change as a result of their connection to each other.