One possible plot could be that after Anakin turns away from the dark side, he and Padme work together to bring a new era of peace and prosperity to the galaxy. They use their combined influence to reform the Senate, improve relations between different planets, and promote the growth of the Jedi Order in a more balanced way.
The Sith could still be antagonists. Even though Anakin has turned away from the dark side, there might be remnants of the Sith order who want to see his rule fail and plunge the galaxy back into chaos. They might try to manipulate events and turn Anakin's own people against him.
Perhaps in the fanfic, Anakin has already fully embraced the dark side when Padme arrives on Mustafar. He shows her his new powers, thinking she will be impressed. But Padme is horrified. She tries to use their bond to reach the old Anakin inside. Meanwhile, the volcanic landscape of Mustafar could add a sense of danger and doom to their encounter, as if the planet itself is a symbol of their relationship falling apart.
Well, Padme could try to fight Anakin to stop him from further embracing the dark side. She might use her diplomatic skills and love for him as weapons. For example, she could tell him how their unborn child would grow up without a father if he continued down this path.
A possible plot is a fanfic where Anakin, in his Darth Vader form, has a vision of a different future where he didn't lose Padme. This vision makes him question his loyalty to the dark side. Meanwhile, Luke and Leia are on a mission to rescue their father from the clutches of the Emperor. The story could be filled with emotional reunions and battles between the light and dark forces.
The fanfic might center around Anakin realizing that he and Padme are in a sort of 'love jail' where they can't fully be together in the open. So he changes, perhaps by becoming more rebellious against the rules that keep them apart. He could start to question the Jedi code more vehemently and strive to break free from the situation for the sake of their love.
They might be shocked by the advanced technology. The future could have flying cars and huge skyscrapers everywhere. Anakin's Jedi skills might be seen in a new light, and Padme could be amazed at how society has evolved in terms of equality and governance.
A possible plot could be that Padme fakes her death as in the original story, but Vader discovers she is alive and pregnant. He then decides to break away from the Empire to start a new life with her and their child, but they must evade the Empire's forces and Vader has to come to terms with his new identity outside of the Sith.
One interesting plot point could be how they navigate their relationship in secret. Since Anakin is a Jedi and Padme is a senator, their relationship is against the Jedi Code. Fanfics might explore how they keep it hidden from others, like sneaking around to meet.
One idea could be focusing on how they prepare for the baby's arrival in secret. Anakin might be building a small crib with his mechanical skills while Padme is knitting baby clothes. Another could be their concerns about the baby's future in a galaxy filled with conflict.
One plot idea could be Ahsoka discovers Anakin's relationship with Padme and has to keep it a secret while also trying to protect them from the Jedi Council's potential discovery. Another could be a story set during a dangerous mission where Anakin and Padme are in peril, and Ahsoka must use her skills to rescue them. A third idea is a fanfic that explores how Ahsoka's presence in Anakin's life affects his relationship with Padme, whether it brings them closer or causes some unforeseen tensions.