Vandal Savage could be a great main villain. Since he has a long - standing history in the Young Justice universe, he could be using the chaos of Endgame to his advantage. He might be secretly manipulating events behind the scenes to further his own goals of world domination.
Vandal Savage is also a possibility. He has a long - standing enmity with the heroes. Maybe he has a new plan that involves using the resurrected heroes for his own evil purposes, or he's trying to stop their resurrection because he fears they'll thwart his future plans.
The Court of Owls would be a great main villain. They are a powerful and secretive organization in Gotham. Their connection to Gotham's history and their shadowy nature could pose a huge threat to both Young Justice and Batgirl. They might be after some meta - human power or trying to take over Gotham in a new way.
It could be Kronos again, but in a more powerful and resurrected form. He might have found a way to come back even stronger after his previous defeat and now has a new plan to take over the world of gods and demigods.
Lex Luthor could be a great villain. He's always interested in taking down the Justice League and its affiliates. He might see the clone as a way to disrupt the Young Justice team and gain an advantage over Wonder Woman.
It could be a future version of an existing villain who has somehow gained even more power. For example, a more advanced and menacing version of the Light. They might be the ones causing the future problems that Future Robin is trying to prevent, and their actions in the present are just the start of their grand plan.
Sportsmaster would be a great villain. He's a skilled fighter and strategist, and he could target the apprentice specifically to get to the Young Justice team. He could also be training his own apprentice to go up against the Young Justice apprentice, creating a rivalry between the two.
Darkseid as a vampire - like being. He could come to Earth and start turning people into his vampire minions. His already formidable powers combined with vampiric abilities would make him a terrifying foe for the Justice League.
Dick Grayson is often a popular character. His leadership skills and complex character make him a central figure in many fanfictions. Artemis is also popular as she has a strong personality and her own set of skills. And Superboy, with his struggle between his human and Kryptonian sides, is frequently explored in post - Endgame fanfiction.
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.
Darkseid would be a great main villain. He's already a powerful and well - known foe in the DC universe. He could be the one who starts collecting the Infinity Stones - like items to gain ultimate power and rule over all universes. His army could also be involved, making it a large - scale war for the Justice League.