One interesting plot could be that Sarah, after being kidnapped, discovers a hidden side of Jareth's kingdom that he has kept secret from everyone. She finds out that there are other captives there who are being held against their will for reasons she has to uncover. As she tries to escape and free the others, she starts to see Jareth in a different light, not just as a captor but as a complex being with his own motives and past traumas.
The story might end with Sarah actually falling in love with Jareth. She decides to stay in his world and they rule the Goblin Kingdom together, bringing a new era of peace and harmony between the human world and the Goblin realm. This would be a very different ending from the more common 'escape and return home' trope.
The relationship might become more intense. With the pregnancy, they have to rely on each other more. Jareth may show his softer side as he takes care of Sarah, and Sarah might start to see him in a new light, beyond the intimidating Goblin King.
A common theme is the exploration of their unspoken connection. Since in the original there's a certain tension between them, fanfictions often expand on that, like in 'The Unspoken Bond' where it's all about their unacknowledged feelings for each other.
One common theme is the power dynamic between them. Since Jareth is a powerful Goblin King and Sarah is a mortal girl, exploring how their relationship develops in terms of power is popular.
Well, aside from Jareth and Sarah, there could also be other characters from the original story that play important roles. For example, the goblins might be characters that are constantly around. They could be either helpful or cause trouble for the newly married couple. Also, Sarah's friends or family from the human world might be in the story. They could be trying to understand her new life with Jareth, or perhaps be worried about her being in the magical world. And who knows, there might be some new magical characters created just for this fanfiction to add more drama and storylines to their married life.
In fanfiction, the idea that they come from two completely different realms, the human world and the Goblin Kingdom, adds to the allure. Their relationship can be seen as a bridge between these two worlds, and how they navigate the cultural and magical differences is fascinating. For example, in some stories, Sarah has to learn the ways of the Goblin Kingdom to be with Jareth, and Jareth has to understand human emotions better. This dynamic makes for a rich and engaging story.
There could be some out there, but a lot of fanfiction platforms have rules against explicit content. You might have better luck looking for more general Jareth and Sarah fanfiction on Archive of Our Own (AO3). It has a large collection, and you can filter by different themes and ratings to find something that suits your taste, but stay away from inappropriate stuff.
They are a popular pairing because of the strong dynamic between them in the original story. Dark Jareth is a mysterious and powerful character, and Sarah is a brave girl. The contrast makes for an interesting relationship in fanfiction.
Yes, there might be some out there. You can try searching on popular fanfiction websites like Archive of Our Own (AO3). They have a large collection of various fanfictions, and you may find some related to Jareth and Sarah in a pregnancy scenario.