There is a fanfiction called 'Love Beyond the Labyrinth' where Sarah, after getting lost again in the labyrinth, discovers a new side of Jareth and they start to explore their romantic feelings. It has really detailed descriptions of their emotions and the magical setting.
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.
One popular one might be 'The Labyrinth of Love' which delves deep into their relationship. It shows how Sarah overcomes her initial fears of Jareth and they start to understand each other on a deeper level.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfiction for various fandoms, including ones about Sarah and Jareth's love.
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.
Love in this fanfiction could be shown as a force that gives them the strength to face the unknown. The pregnancy is full of uncertainties, like what the baby will be like, how they will raise it between two worlds. But their love for each other and the baby gives them the courage to face all these questions. They support each other emotionally, and their love becomes the foundation for building a family, despite all the odds against them.
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.