Another way to develop their relationship is through a shared secret. Maybe they both know something about the Force that the rest of the galaxy doesn't. This secret brings them together in a way that defies their initial roles as Rebel and Sith. They have to communicate and collaborate to keep this secret, and in the process, their relationship evolves from pure hatred to something more nuanced, like a begrudging alliance with a hint of mutual curiosity.
Another way is through the exploration of their family connection. Vader already knows he is Leia's father, but in fanfiction, he could be conflicted about how to handle this knowledge. Leia, on the other hand, could start to suspect something about their relationship. This internal conflict for both of them could drive the development of their relationship. They would still be on opposite sides in terms of the Rebellion and the Empire, but their personal relationship would be evolving in a very interesting and character - consistent way.
A plot could involve Darth Vader having a moment of hesitation when facing Princess Leia. He sees in her a strength and spirit that reminds him of his past self. This moment of doubt could lead to a series of events where he starts to secretly help her in small ways without the Emperor knowing.
The relationship could start with the female stormtrooper showing great combat skills that catch Darth Vader's attention. He then begins to give her more difficult tasks, and in the process, they develop a mutual respect. For example, she could be the only one in her unit to survive an ambush, and Vader sees her determination.
In some fanfiction, their relationship might develop due to shared goals. Let's say they both want to stop a new superweapon being developed by the Empire. Working together towards this common objective forces them to interact more, and they begin to see each other in a new light. Vader might show moments of vulnerability, which Leia starts to pick up on, and she realizes there is more to him than just the menacing Sith Lord. This mutual discovery helps in building a more nuanced relationship between them.
In an alternate universe, Darth Vader could be exiled from the Empire after a disagreement with the Emperor. He then stumbles upon Leia on a remote planet. At first, they are enemies, but as they are forced to survive together against various threats on the planet, they start to understand each other. Leia realizes that Vader is not all bad, and Vader sees in Leia a determination that he once had. This understanding could lead to a slow transformation of their relationship from hostility to friendship, and eventually, they could work together to try and bring about a new order in the galaxy, one that is not dominated by the Empire or the Rebellion but a more balanced one.
There might be some out there that are not overly adult - themed. However, in the official Star Wars lore, their relationship is one of enmity. Fanfiction that doesn't cross into inappropriate territory could explore different aspects like an alternate universe where they have to work together against a greater threat, perhaps with Leia trying to understand Vader's fall to the dark side in a more empathetic way.
They could start by sharing more of their cultures. Ariel could teach Eric about the different species in the ocean, and Eric could introduce Ariel to the ways of his land. This would deepen their understanding and love for each other.
One idea could be that Darth Vader discovers his connection to Leia earlier and secretly tries to protect her from the Emperor's more sinister plans. He might use his position within the Empire to subtly divert threats away from her.
Explore the idea of redemption through their relationship. Vader's ultimate act of redemption in the original story was related to Luke, but in fanfiction, it could be Leia who also plays a part in that journey. Maybe Leia's unwavering belief in the good is what makes Vader start to question his own path more seriously, without changing who he is fundamentally as a powerful but conflicted Sith.
Their relationship can be explored by showing their training sessions together. Since Luke is more trained in the Force, he could teach Leia some advanced techniques, and in the process, they could have deep conversations about their future roles in the galaxy.