Lex Luthor is often a main antagonist. He might be the one who sets up the situation where Superman needs to be saved, like creating a powerful weapon or a devious plan to weaken Superman.
There could also be a new, original villain created specifically for the fanfic. This villain could have a unique power or ability that allows them to target Superman. For example, a villain who can manipulate Superman's powers against him, and Batman has to figure out the villain's weakness and save Superman before it's too late.
It could be Lex Luthor. He's always been a big threat to the Justice League and would be a likely choice as the main antagonist in a story where Batman has to save the League.
The main antagonists are Lex Luthor. He's always been a foe to Superman and in this graphic novel, he manages to turn public opinion against both Superman and Batman. His devious plans and his ability to manipulate the situation make him a formidable antagonist in the story.
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.
The main antagonists could be a combination of forces. Besides Voldemort and his Death Eaters, there could be some anti - alien groups from the Muggle world who discover Harry's true identity. They fear his power and try to hunt him down. This would put Harry in a difficult position as he has to protect not only the wizarding world but also himself from these new threats that are coming at him from different directions.
One possible antagonist could be Orochimaru. He has always been interested in Naruto for his Nine - Tails power, so it would make sense for him to be a threat that Kakashi has to protect Naruto from.
A theme that frequently appears is redemption. Sometimes Batman might be in a situation because of a mistake he made, and Superman saving him is a form of redemption. Also, the theme of heroism is prominent. Superman's act of saving Batman reaffirms their status as heroes and shows their selfless nature towards each other.
It could be some dark magic users from Lucy's world who see her as a threat. They might be trying to stop her from using her magic for good.
The Joker could be an antagonist even when Batman becomes a god. He might see Batman's new status as a challenge and try to bring him down to his level through his usual chaotic and insane methods. Maybe he'd try to drive Batman mad with the idea that his new powers make him no better than the villains he fights.
When Batman saves Superman, it could make him more confident in his own abilities outside of his normal Gotham - based operations. He realizes that his skills as a detective, strategist, and fighter are not just useful in his own city but can be applied on a much larger scale. This could lead to him being more involved in Justice League - related matters and having a more positive outlook on working with super - powered individuals in general. He might also become more empathetic as he gets to understand Superman's vulnerabilities better.
There might be a new villain or a previously known enemy who has set a trap for Tony in Siberia. This antagonist could be a powerful figure with their own motives, like wanting to take over Stark Industries or gain access to Tony's Iron Man suits. Natasha would then have to face this antagonist to save Tony. They could have their own army or special powers that make the rescue mission more difficult for Natasha.