One interesting storyline could be where Superman and X - 23 team up to fight a common enemy like Darkseid or some other powerful villain. X - 23's combat skills and Superman's superpowers would make for an exciting combination in battle.
Another storyline could involve X - 23 being in a world she doesn't understand, and Superman helps her adapt. Since Superman is an icon of hope and justice, he could teach X - 23 about using her powers for good in a more positive way. He could show her that there's more to life than just being a weapon. This could lead to a character - growth arc for X - 23.
A third storyline might be a love - triangle situation where X - 23, Superman, and another character like Lois Lane are involved. This would create a lot of drama and conflict. X - 23's cold and stoic nature would contrast with Lois's more human and emotional side, and Superman would be caught in the middle, trying to balance his feelings for both of them while still being a hero.
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.
One interesting storyline could be Superman and She - Hulk teaming up to fight a powerful alien threat that is immune to most of their individual powers. They have to combine their strength and unique abilities, like Superman's flight and heat vision with She - Hulk's brute strength and legal expertise (since she's also a lawyer in her civilian identity) to outwit the alien. Another could be a story where they are in an alternate universe where they are the only two heroes left and they have to repopulate the earth with super - powered beings. This would be a really unique take on their relationship.
One interesting storyline could be Supergirl and Superman teaming up to face a powerful new villain from another dimension. They would have to combine their powers and learn to work in perfect sync to defeat this threat.
Maybe a storyline where Mala has some unique powers that Superman doesn't have, and they have to team up to face a powerful enemy. Their combined efforts would make for an exciting adventure, with each using their own strengths to overcome obstacles.
One interesting storyline could be a team - up against a powerful new villain that neither can defeat alone. They have to combine their unique skills, like Superman's super strength and Batman's detective skills.
In some Mpreg fanfictions, there could be a storyline where Batman and Superman have to figure out the genetics of their unborn child. Given their different origins - one being a Kryptonian and the other a human with no super - powers originally - it would be fascinating to see how their child might inherit or combine their abilities. There could also be a plot where they have to deal with time - traveling villains who try to prevent the birth of their child for some evil purpose.
A cool story could be about them having a friendly competition to see who can solve a series of mysterious crimes in the other's city first. Along the way, they learn more about each other's methods and develop a deeper respect for one another. This also gives an opportunity to explore the different atmospheres and challenges of Gotham and Metropolis.
One interesting storyline could be a young Superman and Batman teaming up for the first time in a kid - friendly adventure. Maybe they are trying to stop a group of small - time villains who are stealing toys from all the kids in Gotham. They use their unique powers and detective skills respectively to track down the bad guys and return the toys.
One interesting storyline could be them teaming up against a new supervillain that neither can defeat alone. They combine their unique skills, Superman's super strength and flight with Batman's detective skills and gadgets.
A great storyline is when a powerful ancient artifact is discovered. This artifact has the potential to destroy the world if misused. Superman and Wonder Woman are both drawn to protect it. However, they face many challenges as various villains also want to get their hands on it. Along the way, they learn more about each other's pasts and how their powers originated, which deepens their friendship and partnership.