1. Injustice. This idea is very loosely based on the new game Injustice. You do not need to have played Injustice for this, but a little knowledge of the DC universe would help. Essentially, I want to do a story that starts with Superman being captured by Darkseid and undergoing a brainwashing process that brings out the darkest elements of his personality. He returns to earth at the forefront of an invasion, cruel and completely heartless. Your character (I'm picturing Wonder Woman, but I could go with an superheroine) opposes him and eventually breaks him free of the mind control. They stop the invasion, but not before much of Metropolis is destroyed. In a fit of rage, Clark kills Darkseid, but given what has happened, the people of earth are supportive of this.
In the weeks that follow, the world sees a much different Superman. Motivated by grief and still somewhat influenced by the mind control process, he starts exerting his influence more and more. He enforces ceasefires between countries, helps rebels overthrow dictators, and frightens criminals into behaving.
Essentially, I want to draw out a process where Clark becomes more and more proactive in the use of his powers, until he eventually decides that the only way to save humanity from itself is to rule it with an iron fist. Your character could be his loyal general, or she could slowly start to oppose what he is doing, only to be subjugated by him. Either way, I see it developing into a dom/sub relationship. Thanks to the brainwashing, he discovers that he likes holding power over your character. Perhaps Darkseid's brainwashing tech still exists, and he uses to to tweak her mind a little, making her crave feelings of helplessness or long to be controlled by him. There are a lot of directions this could go. She could even be captured during the initial invasion and humiliated a bit before managing to snap him out of it. I'm very flexible in planning out a story that will involve my kinks and the kinks of my partner. I just really like the idea of playing against a hero who is publicly a powerful leader in his government, but secretly submissive and eager to please the new ruler.