One possible fanfic could be about Mika and Krul forming an unexpected alliance. Maybe they have to work together to face a common enemy that threatens both of their existences. In this story, Mika might start to see Krul in a different light, not just as a vampire but as someone with her own motives and desires.
There's also 'Fate's Entwined Threads' which is quite good. It shows how their relationship might develop over time, with interesting character interactions and a plot that keeps you hooked. The author does a great job of staying true to the characters' personalities from the original work.
The 'owari no seraph' universe itself is quite dark and filled with complex lore. Fanfics about Krul and Mika allow fans to explore different aspects of this universe that may not be fully explored in the original work. It's also about the way they interact, with Krul often being in a position of power and Mika having to navigate that relationship while also dealing with his own issues.
A plot idea could be that Mika discovers a secret about Krul's past that changes his perception of her completely. This could lead to him trying to protect her from those who would use the secret against her.
One common theme is survival. Given the post - apocalyptic setting of 'Owari no Seraph', many oc fanfictions focus on how the original characters endure and fight for their lives. Another theme is friendship. The oc often forms strong bonds with the existing characters, like Yuuichiro and Mikaela. For example, in some fanfictions, the oc and Yuuichiro help each other in battles and grow to trust each other deeply.
The Owari no Seraph Michaela novels often explore the theme of survival. In a post - apocalyptic world, the characters have to fight for their lives against various threats. There's also a strong theme of redemption. Some characters carry the burden of past mistakes and strive to make amends. Additionally, power and its consequences are explored. As the characters gain power, they also face the risk of being corrupted by it.
The ending of Owari no Seraph light novel is quite intense and leaves some room for interpretation. The main characters face significant challenges and their fates are resolved in a way that surprises many readers.