A more complex interaction could be when they face a moral dilemma. For example, Superman has his strict code of not killing, while Robin, having grown up in the darker world of Gotham, might be more inclined towards harsher methods at times. This could lead to some intense discussions between them. But in the end, they could learn from each other. Superman could show Robin that there are always alternatives to violence, and Robin could teach Superman about the importance of being adaptable in different situations.
The OC could be an alien with a different kind of power set that is a threat to Superman. However, instead of fighting, the OC tries to reason with Superman. They show him the consequences of his actions through their own experiences on their home planet where a similar situation occurred, which might make Superman re - evaluate his path.
First, create a strong conflict. Since it's Dark Superman, you could have the reader oppose his extreme ways. For example, the reader could be part of a resistance group. Second, develop the relationship between the reader and Dark Superman gradually. Maybe they start as enemies but find common ground. Third, add some twists. For instance, the reader discovers a secret about Dark Superman that changes their perspective. Keep the plot moving forward and don't let it get stagnant.
One common theme is the struggle between power and morality. In the Injustice universe, Dark Superman has a very different view on justice compared to the traditional Superman. In an x reader fanfiction, the reader often has to navigate this new moral landscape created by Dark Superman's actions. Another theme could be redemption. Maybe the reader tries to bring back the good in Dark Superman. Also, themes of love and loyalty can be explored as the reader might be in a relationship with Dark Superman and has to decide whether to support his extreme methods or try to change him.
Definitely Iron Man and Superman. Iron Man with his technological genius would be fascinated by Superman's alien biology and powers. He might try to create some sort of device to enhance or analyze Superman. And Superman would be interested in Tony Stark's inventions.
The OC could have a unique power that directly counteracts Superman's. For example, if Superman has his heat vision, the OC could have the ability to absorb and redirect energy. This would make their encounters very intense and give the OC an edge in battles.