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.
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.
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.
Their relationship can be developed through flashbacks. For example, showing memories of Anakin and Ahsoka's time as master and apprentice, and then contrasting it with their current hostile stances as Ahsoka and Darth Vader. This can add depth to their interactions.
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.
Well, their relationship could be developed through their mutual obsession. Michael is obsessed with hunting Laurie, and Laurie is obsessed with stopping Michael. In the fanfiction, this could be explored more deeply. For example, Laurie could start to predict Michael's moves based on his past actions and her knowledge of him, which would make their encounters more intense.
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.
Well, their relationship could start with mutual respect for each other's skills. Shan Yu respects Mulan's military tactics and Mulan respects Shan Yu's leadership among his men. This respect could gradually turn into a grudging admiration. For example, Shan Yu might admire how Mulan overcame so many obstacles as a woman in a man's army, and Mulan could admire Shan Yu's determination.
They could start as friends. Romilda could offer to help Harry with his schoolwork, especially in areas she's good at. Since Harry is a kind person, he would accept the help. As they spend time together, they gradually realize they have feelings for each other. Harry's loyalty and Romilda's determination can still be there, just in the context of their relationship growth.
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.