You need to consider their different backgrounds. Superman has his Kryptonian heritage and his experiences in the Injustice 2 world. The reader could be from a more ordinary background. Show how these differences can both create friction and also draw them together. For instance, the reader might teach Superman about the simple joys of Earth life that he's forgotten in his pursuit of justice, which deepens their connection.
In the Injustice 2 Batman ending story, Batman manages to defeat Superman. He restores order to a world that has been torn apart by the chaos of the Injustice regime. With Superman out of the way, Batman is able to start rebuilding society, bringing back justice and the rule of law. He also has to deal with the remaining super - villains and ensure that they are locked away safely.
Obviously, Batman and Superman are the most prominent. Their complex relationship, which is a mix of friendship, rivalry, and ideological differences, makes them central to any fanfiction in this universe. However, characters like Wonder Woman also often play important roles. In some fanfictions, she may be caught in the middle of the Batman - Superman conflict, trying to mediate or having her own stance on the events that unfold.
One interesting storyline could be a redemption arc for Superman. In the Injustice universe, Superman has gone down a dark path. A fanfiction might explore how Batman tries to bring back the good in him, with lots of emotional confrontations and moral debates between the two.
The canon ending of Injustice 2 is often determined by the game's storyline and the decisions made by the developers to shape the overall narrative. Usually, it's the one that ties in with future installments or related media.
One idea could be a story where the reader is a new hero in the Injustice 2 universe. Superman, initially suspicious, starts to see the good in the reader. As they team up against common threats, a bond forms between them. Maybe the reader has a unique power that Superman comes to rely on.
First, decide on the OC's origin. Are they from Earth in the Injustice universe, or from somewhere else? If they're from Earth, what kind of upbringing did they have? For example, if they grew up in a war - torn area because of Dark Superman's actions, it would shape their character. Next, think about their motivation. Is it revenge against Dark Superman, or a desire to restore true justice? Also, give them some unique traits, like a special skill or a distinct personality quirk. For instance, they could be extremely stubborn in their beliefs, which would lead them to take on Dark Superman even when others won't.
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.