One common theme is the struggle against their different natures. Kol is a vampire with a dark past and Davina is a powerful witch. Their relationship often has to overcome the challenges posed by their respective supernatural identities.
One possible plot could be that Kol turns Davina into a vampire out of love. He doesn't want to lose her to mortality. Davina, at first, is horrified as she has always been on the side of the witches. But as she starts to experience her new vampire powers, she begins to see the world in a different light. She and Kol then have to face the wrath of the witch community who see her transformation as a betrayal.
Their relationship would become more intense. As a vampire, Davina would have new emotions and instincts. Kol, who has more experience as a vampire, would have to guide her. This could lead to some power struggles as Davina is used to being in control. But it could also bring them closer as they face new challenges together.
For Davina, she would become more cautious. As a mother - to - be, she would no longer take risks as easily. Her focus would shift from just her own powers and place in the supernatural world to protecting her unborn child. Kol, being the father, would likely become more responsible. He has a bit of a wild streak, but the pregnancy would make him think twice before getting into dangerous situations.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions on various topics, including Kol and Davina as soulmates. Just use the search function and type in 'Kol and Davina soulmates' and you'll likely find many interesting stories.
One possible plot could be that Davina and Kol discover the pregnancy unexpectedly. They might face many challenges. For example, they would have to deal with the reactions of their families and friends in the supernatural world. Some might be happy, while others could be worried about the implications of a half - witch, half - vampire baby. Also, they would need to protect the unborn child from the many threats in their world.
Kol could be the father. Given their relationship in The Originals, it would be a likely scenario. Their relationship was complex and a pregnancy would add another layer of depth to it.
One common theme is the power struggle. Given Klaus's powerful vampire nature and Davina's strong magic, there's often a play of power between them. Another theme is redemption. Klaus is often seen as a villain, and in some fanfictions, Davina helps him find a path to redemption.
There's 'Kol Mikaelson: The Lost Years'. This one is about Kol's unaccounted - for years. It's full of adventure and mystery. The author has clearly done their research on Kol's character and the Vampire Diaries universe in general. It's a great read for those who want to explore more about Kol's backstory and how it might have shaped the character we know in the show.
Kol is impulsive and often acts on his desires. This would likely lead to him getting into trouble during time travel. For example, he might not be able to resist the urge to feed on someone in a time when vampires are not known, which could expose their presence. Bonnie, on the other hand, is more cautious. She would be constantly worried about the consequences of their actions on the timeline. So, their time travel would be a balance between Kol's recklessness and Bonnie's caution.