Well, perhaps they reunite in a world that is a blend of the real world and the Labyrinth. Jareth could be seeking Sarah's help to save the Labyrinth, which has fallen into some sort of chaos. Sarah, having a soft spot for the place despite the hardships she faced there, might be willing to assist. Their relationship could evolve from enmity and fascination to something more like comradeship, with a hint of the old romantic tension still lingering in the air. And as they work together, they start to understand each other better.
In a fanfiction scenario of their reunion, it could be that Jareth has changed. He may have realized the error of his ways in trying to control Sarah before. So when they meet again, he is more gentle and apologetic. Sarah, being the kind - hearted person she is, forgives him. They then begin to explore the Labyrinth together, not as captor and captive, but as equals. They discover new parts of the Labyrinth and along the way, their bond deepens into a true friendship or even something more, with Jareth using his magic to show Sarah the wonders of his world.
One possibility is that there would be a lot of unresolved tension between them. Jareth, with his mysterious allure, might initially be cold towards Sarah, but deep down still hold feelings for her. Sarah, on the other hand, would likely be both fearful and curious. She might be more confident this time around, having grown since her first encounter in the labyrinth. Their reunion could spark a new adventure, with Jareth trying to tempt Sarah once again, but this time she could be more resistant, leading to a power struggle of sorts.