Well, in batman vs justice league fanfiction, characters like Nightwing and Oracle are prime candidates to side with Batman. Nightwing was Batman's first Robin and has a loyalty to him. Oracle, with her intelligence and access to information, would back Batman if she believed his cause was just. Additionally, characters like Batgirl could also side with Batman, especially if she thought the Justice League was overstepping its bounds.
In addition to Batman, Superman, and Wonder Woman, The Flash could also be a main character. His super - speed would add an element of quick - paced action and his humorous personality would provide some levity. Aquaman, with his connection to the ocean and his trident, is also likely to be a main character. He can bring in a different perspective, especially when it comes to underwater or environmental threats. Green Lantern, with his power ring and willpower, would also be important as he has unique abilities that can be used in various ways to fight threats.
Obviously, Batman would be one of the main characters. And of course, the other members of the Justice League like Superman, Wonder Woman, Flash, Green Lantern, etc. would also be important. There might also be some villains that are central to the story, such as Lex Luthor who could be plotting against the newly - formed alliance of Batman and the Justice League.
Batman is obviously one of the most popular. He's a complex character with a lot of depth, and in a 'vs' scenario with the Justice League, his skills and intellect are put to the extreme test. Superman is also popular. His god - like powers in contrast to Batman's more human abilities create a great dynamic. Wonder Woman is another favorite. Her strong moral code and combat skills add an interesting element to the story when she's involved in the conflict between Batman and the League.
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.
Obviously, Batman's kids would be main characters. Depending on which kids are included, it could be Robin or Batgirl, etc. And of course, the members of the Justice League like Superman, Wonder Woman, Flash, etc. would also be main characters as they are meeting Batman's kids.
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.
The Joker is a very likely character. He has a long - standing vendetta against Batman and might have finally found a way to almost kill him. He could be working with some other powerful villains or using some new, deadly weapon or toxin.
Green Lantern could be another significant character. He could introduce the Batman sons to the idea of using willpower and the power of the ring. Aquaman might be involved too, perhaps showing the sons the importance of protecting the oceans. And Martian Manhunter, with his vast powers and wisdom, could offer unique insights to the sons and help them understand their place in a bigger universe.
For the Justice League, Superman, Batman, and Wonder Woman are almost always main characters. Their powers and leadership are crucial. From the Seven, Homelander would likely be a central figure as he is one of the most powerful and complex in the group.
Batman's sacrifice would likely have a huge impact on the Justice League. It could make them more determined to uphold justice in his memory. They might also become more cautious as they've lost a great strategist.