One possible storyline could be how they deal with the challenges of impending parenthood while still being involved in the galactic struggle. For example, they might have to find a safe place to raise their child away from the Empire's reach.
In time travel situations, they may have to rely on each other in ways they haven't before. For example, if they travel to a time when the Force is not well - understood, Luke may have to depend on Han's non - Force - related skills more. Leia might find herself in situations where she has to mediate between Han and Luke more often, strengthening their bond as a trio. Their relationships would grow deeper as they face new challenges together.
If they time travel, their relationships might become more complex. For example, if they go back in time to before they met, they might initially not trust each other as they do in their own time. Han might be more distrustful of Luke's Jedi abilities, and Leia might not see Han as the hero she knows him to be. But as they face challenges together in the past, they could form deeper bonds in different ways than they did originally.
They could time travel to a parallel universe where the roles are reversed. In this universe, Luke is a Sith, Leia is a bounty hunter, and Han is a political leader. They are shocked by these changes but soon realize that they need to find a way back to their own universe. Along the way, they learn about the different choices that led to these alternate identities. Luke sees how easy it was for him to turn to the dark side in this universe, which makes him more determined to stay on the path of the light in his own world.
The pregnancy might change the way other characters interact with them. Allies could be more protective, while enemies might see it as a new vulnerability to exploit. For example, the Empire could try to use the pregnancy as a way to blackmail or manipulate them. It also gives fanfiction writers a chance to explore different aspects of the characters' personalities as they adapt to this new situation.
One idea could be that they travel back to the time before the fall of the Republic. Han could try to use his smuggler skills to prevent certain events that led to the rise of the Empire. Luke might seek to train with the Jedi of that era, and Leia could try to influence galactic politics to avoid the chaos that ensued.
In some fanfiction, their relationship starts with pure enmity. Jabba sees Leia as a trophy and a means to gain power over the Rebellion. But as the story progresses, Leia might start to understand Jabba's motives more deeply, and Jabba might begin to respect Leia's tenacity.
One possible fanfiction could be about Leia's attempt to outwit Jabba in a political negotiation rather than the more common scenarios of her being in captivity. She could use her diplomatic skills and knowledge of the galaxy's power dynamics to gain an upper hand.
The idea of legacy is also prevalent. As Anakin was a very powerful Jedi - turned - Sith and Leia is a strong figure in the Rebel Alliance, their combined legacy is a rich area for fanfiction. Writers might explore how Leia could inherit some of Anakin's skills or how she could redefine the Skywalker legacy in a positive light.
There could be a fanfiction where Luke and Leia explore their relationship as siblings in more depth. For example, they could have heart - to - heart conversations about their parents and how their upbringing affected them. This would add more emotional depth to their characters in the fan - created story.