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.
A plotline might involve them going on adventures together outside of their usual work - related tasks. They could discover new aspects of each other's personalities and grow closer as a result. Also, there could be a plotline where they face opposition from other characters in the Helluva Boss universe who don't approve of their relationship.
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.
The contrast between their characters. Stolas is refined and powerful, while Blitz is rough around the edges and scrappy. This creates a dynamic that fans find interesting to explore in fanfiction.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions across various fandoms, and there are likely many Stolas x Blitz fanfictions there. Just use the search function on the site.
A common theme is friendship. Many fanfictions focus on how Red and Stella become friends and how that friendship enriches their lives. For example, they might share a common goal and work together towards it, strengthening their bond in the process.
One common theme is love and romance. Since they are a couple, most fanfictions explore their relationship in different ways, like how they fall in love or keep their love alive.
Since I'm not well - versed in Stella x Tsuribari fanfic specifically, it's hard to say for sure. But in general, themes like friendship, adventure, and self - discovery could be common. Maybe there are also themes related to the unique relationship between Stella and Tsuribari.