Another theme could be their exploration of new places or cultures. They could embark on a journey together, learning about different languages, traditions and making new friends along the way. This kind of fanfiction would be positive and engaging without any inappropriate content.
There may be some fan - made works out there, but engaging with sexual fanfiction is not in good taste. It's important to respect the source material and the characters. You could try to find fanfiction that focuses on their growth as individuals, like how Klaus becomes more confident or Isadora discovers new talents.
One popular theme is the forbidden love aspect. Given their different backgrounds in their respective fictional universes, a relationship between Klaus and Bella would be seen as unconventional. For example, Klaus being a vampire from one world and Bella being involved with vampires in another way in her world.
Another theme is the protection of the unborn child. Given the dangerous supernatural world they live in, both Bonnie and Klaus are usually depicted as being extremely protective. They will go to great lengths to keep their baby safe from all the threats out there.
Another theme might be Klaus's reaction to the pregnancy. Some fictions explore whether he is protective, indifferent, or even hostile at first. His complex character adds a lot of depth to this kind of storyline.
One common theme could be protection. With Klaus being a powerful but also very much hunted character, protecting the baby from his enemies would be crucial. Another theme might be family. How the baby fits into the already complex family structure of the Originals.
One popular theme is romance. Writers often create relationships between a female character and Klaus, exploring the complex dynamic between them. Another theme is redemption. Since Klaus has a rather dark past, fanfiction might explore his journey towards redemption with the help or influence of the female character.
A sense of hope is a common theme too. The pregnancy can symbolize a new start for them. It can be a chance to break away from their past mistakes or a way to build a better future for their family. It also gives an opportunity for character growth, especially for Klaus who has a complex character.
One common theme is friendship. Often, the oc forms a deep and unexpected friendship with Klaus. Another theme is redemption. The oc might be involved in Klaus's journey towards redemption. For example, in some fanfictions, the oc helps Klaus face his past wrongs.
A theme that frequently shows up is redemption. Klaus, being a complex and often villainous character, may be on a path to redemption in some fanfictions, with Stefan either helping or hindering. Also, power dynamics play a role. Klaus is very powerful, and Stefan has his own set of abilities, so how they interact in terms of power is a common theme. For example, in some stories, Stefan has to find ways to level the playing field when dealing with Klaus.
One common theme is redemption. When Klaus is hurt, it can be a starting point for him to seek redemption for his past wrongs. Another theme is vulnerability. It shows a side of Klaus that we don't always see, as he's usually portrayed as strong. For example, in some fanfictions, his hurt might make him more open to forming new relationships or mending old ones.