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.
Another possible storyline is that Superman and Batman are turned into kids themselves in a magical mishap. They have to rely on their child versions of themselves to figure out how to reverse the spell. Along the way, they learn a lot about what it's like to be a normal kid while still having their heroic instincts, which makes for a fun and educational story for the readers.
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 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.
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 possible storyline could be that Batman has to use his detective skills to find out a hidden threat to Superman before saving him. Maybe there's a kryptonite - based weapon that only Batman can detect and disable.
One interesting storyline could be their child having to hide their powers in a normal school. They'd struggle to fit in while also having the responsibility of their superhero heritage. Another could be the child getting kidnapped by a villain who wants to use their unique genetic makeup for evil.
A really interesting plot could be that the child is kidnapped by an old enemy of either Wonder Woman or Batman. The kidnapper wants to use the child to get revenge on the heroes. The child then has to use their wits and the powers they've inherited to escape. Along the way, they might make new friends or discover secrets about their parents' pasts that help them in their journey.
One popular storyline could be them teaming up against a common enemy that is too powerful for either of them alone. For example, a new super - villain who has the power to manipulate time. They have to combine their skills, Batman's detective skills and Superman's super strength and flight, to figure out how to defeat this villain. Another storyline might involve them having a falling - out over their different methods of protecting the city, and then having to reconcile in order to save the world from a cataclysmic event. There could also be a story where they discover they are long - lost brothers in a more in - depth way, exploring their family history and how it impacts their relationship.
One interesting storyline could be a reimagining of Batman's first encounter with the Joker in the 1939 fanfiction context. Maybe the Joker starts as a small - time criminal and Batman has to figure out his true motives early on.