Darkseid could also be the main villain. He's a powerful and menacing threat from another world. His arrival on Earth could force Superman and Batman to join forces. Darkseid's goal of conquering Earth would be a huge challenge for both heroes, as Superman has to face Darkseid's physical power and Batman has to figure out his plans using his detective skills.
Lex Luthor would be a great choice. He's Superman's long - time nemesis. He could form an alliance with some of Batman's foes like the Joker. Luthor could use his intelligence and resources to create a plan that only Superman and Batman together could stop.
The Riddler could be an interesting main villain in this fanfiction. He could create puzzles and riddles that are so complex that they require both Superman's powers and Batman's intellect to solve. He could be working for a bigger, unknown force, which adds more mystery to the story. Maybe he sets up traps all over Metropolis and Gotham, and Superman and Batman have to race against time to stop him before the cities are in total chaos.
One possible villain could be Bizarro. Bizarro is always causing chaos and with Superboy and Superman together, it would be an interesting fight. Bizarro's reversed logic and powers would make it a tough battle for them.
Lex Luthor would be a great villain. He could be plotting to destroy Superman once and for all while also trying to outsmart Batman. His intelligence and resources make him a formidable foe for both heroes.
Maybe the Riddler. He could set up a series of elaborate riddles that not only challenge Batman but also the curious kid OC. The kid's fresh perspective might actually help Batman solve the riddles faster and catch the Riddler before he executes his grand plan.
Ra's al Ghul could be a great choice. He might see the daughter as a potential heir or a threat to his plans for Gotham. He could try to manipulate her or use her against Batman. This would force Batman to not only protect his daughter but also try to make her understand the true nature of Ra's al Ghul.
Trigon, Raven's father, could also be the main villain. He might see Superman as a new obstacle in his path to spreading chaos and destruction. His demonic powers would be a huge challenge for both Raven, who has a complex relationship with him, and Superman, who has never faced such a dark and powerful entity from another dimension.
It could be someone like Ra's al Ghul. He is a powerful and complex villain that both Batman and The Shadow would have a hard time dealing with. His global influence and long - life plans would pose a great threat to their efforts to protect Gotham and the world.
Lex Luthor could be a likely main villain. He hates Superman and would see Damian as a tool to use against him. He might manipulate Damian into believing Superman is a threat to Gotham, causing Damian to attack Superman first.
Darkseid could be the main villain. Imagine Darkseid learning of Superman's ability to time - travel and using it to his advantage. He could try to invade different time periods to gain more power and resources. Superman would have to fight him across time and space to protect all of existence.
Brainiac could also work well. He is an android with an insatiable thirst for knowledge and a penchant for collecting and destroying planets. In a crossover, he could target Earth, specifically Gotham and Metropolis. His advanced technology would pose a major challenge to both Superman and Batman. Superman's super - powers would be put to the test against Brainiac's powerful defenses, and Batman would need to use his wits to find a weakness in Brainiac's systems, leading to an exciting and complex story.
Lex Luthor would be a great antagonist. He could play on the differences between Batman and Superman, trying to turn them against each other while he pursues his own evil plans. Maybe he creates a situation where it seems like Superman is a threat to Gotham, causing Batman to be on high alert.