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.
Sometimes, an old enemy of Nightwing might resurface and take on the Justice League as a way to get to him. For example, a crime lord from Nightwing's past who has amassed new and dangerous technology and uses it to take down the League, thinking that Nightwing will come running to save them.
Darkseid is another possible villain. He is extremely powerful. He could send his minions to attack the Justice League first, weakening them, and then use some sort of magic or technology to trap them. Batman would then have to find a way to free the League from Darkseid's clutches, perhaps by finding a weakness in Darkseid's power source.
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.
The plot could involve Batman saving the Justice League from a situation where they are physically trapped. Maybe they are in some sort of energy - based prison. Batman, with his detective skills, would first figure out how the prison works. Then, he'd use his gadgets like his grappling hook and batarangs to break through the prison's defenses and free his comrades.
Well, in this fanfic, perhaps the Justice League has been poisoned or mind - controlled. Batman, being the resourceful hero he is, might first analyze the situation carefully. He could then track down the source of the problem, whether it's a new super - villain or an old enemy with a new trick. He'd likely use his Bat - computer to find a cure or counter - measure and then fight his way through any minions to administer it to the League members, saving them from certain doom.
Lex Luthor could be a likely villain. He's always devising plans to take down the Justice League, and this time he might have come up with a scheme that almost succeeds, until Batman steps in. For instance, he could create a super - weapon that targets the unique powers of each Justice League member.
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.
Some members might be shocked. Wonder Woman, for instance, who respects Batman's skills and tenacity, would likely be very disappointed. She might try to persuade him to stay, emphasizing the importance of his presence in the team. Superman, on the other hand, might initially be understanding, as he knows Batman has his own way of doing things. But he would also be concerned about the power vacuum that Batman's departure would create. Aquaman might be more indifferent, thinking that Batman was always an outsider to some extent, with his focus mainly on land - based issues.
Batman is a master strategist. He would study the weaknesses of each Justice League member beforehand. For example, he knows that Superman has a weakness to Kryptonite. He would also use his advanced technology and gadgets to even the odds. His bat - suit is equipped with various tools that can be used against different opponents.
In a fanfic where Batman is injured, it could lead to many interesting plot developments. Maybe the other Justice League members have to step up their game. Superman might take on a more protective role over Gotham in Batman's absence. Wonder Woman could be the one to lead the investigations that Batman usually handles. This kind of injury to Batman can create a shift in the power dynamics within the Justice League.