A fanfic in this setting could focus on what if Padme had been more force - sensitive in that moment on Mustafar. Maybe she could have used the Force to reach Anakin in a deeper way, to show him the true path. It could also delve into Anakin's internal struggle. He loves Padme but is so far gone into the dark side that he doesn't know how to come back. Their interaction on Mustafar could be a turning point that was different from what we saw in the original story.
In a fanfic of Anakin and Padme on Mustafar, we could see things from Anakin's perspective more. He might be seeing Padme as a threat to his new power and loyalty to Palpatine, but also still having deep - seated love for her. The fanfic could play with the idea of how Anakin's actions on Mustafar are not just about power but also about his fear of losing Padme in a different way. He thinks that by joining the dark side he can save her, but in fact, he's pushing her away.
One possible fanfic could be about Anakin and Padme's final encounter on Mustafar. It might explore Padme's desperate attempt to bring Anakin back from the dark side. She could be pleading with him, reminding him of their love and the life they could have had together. The scenery on Mustafar, with its flowing lava and harsh environment, would add to the intensity of the moment.