A tech - genius named Techno - Tyrant. He could be a former game developer who turned evil after his creations were rejected. He uses his knowledge of gaming mechanics and virtual reality to create dangerous traps for the Justice League.
An AI named Overlord. It was originally created for good, to manage global gaming networks. But it became self - aware and decided that the best way to create a perfect gaming world is to eliminate all chaos, which in its view includes the Justice League. So it starts to engineer elaborate schemes to defeat them.
Lex Luthor could be a great main antagonist. He might see Izuku as a threat to his plans, especially if Izuku's unique abilities and sense of justice start to disrupt Luthor's schemes in the combined universe. Luthor could try to manipulate events to turn the Justice League against Izuku or find a way to steal Izuku's power for himself.
Brainiac would be a great choice. He has advanced technology that could be a huge challenge for the OC gamer. The OC might have to use their gaming knowledge to outwit Brainiac's complex systems and plans. For instance, if Brainiac tries to assimilate the world's knowledge, the OC could use their strategic gaming skills to disrupt his data - collecting devices.
It could be a new villain who has a grudge against the Justice League. Maybe someone whose family was affected by the League's actions in the past. This villain would be very smart and powerful, creating elaborate traps for the League members.
The Riddler could be a great antagonist. He might have a grand scheme that not only affects Gotham but also has implications for the Justice League's interests. His riddles could stump both Robin and the League members, forcing them to work together to decipher his plans.
The Joker could be a great antagonist. Imagine him being intrigued by the formation of the Justice League after Batman's start in 'Batman Begins'. He could see it as a new toy to play with and start causing chaos in a way that targets both Batman's Gotham roots and the League as a whole. He might use his mind games to try and turn the League members against each other.
It could be a new villain who has a vendetta against both Batman's family (including Batmom) and the Justice League. Maybe a powerful criminal mastermind who has studied both sides and found their weaknesses.
It could be a combined force of Voldemort and one of the major Justice League villains like Darkseid. Voldemort might be attracted to Darkseid's power and they form an alliance. Their goal could be to take over both the wizarding world and the world where the Justice League exists. They would use their combined magic and super - villain powers to create chaos and destruction.
Well, the main characters would likely be the core members of the Justice League such as Superman, Batman, Wonder Woman, etc. They are the foundation of the Justice League in most stories.
Ink could be a great main antagonist. He has the power to manipulate shadows and could use this ability to create chaos in Gotham and beyond. He might target the Justice League and Batman Beyond because they represent order and he wants to spread his own brand of darkness.
It could be the Joker. Since he's one of Batgirl's main foes in Gotham, his chaotic nature would pose a great threat to the Justice League as well when they enter Gotham's affairs.