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.
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.
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.
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.
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.
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.
Perhaps a member of the Justice League, accidentally. For instance, during a fight where powers go out of control. Let's say Flash runs too fast and accidentally knocks Batman into a sharp object, causing a fatal injury. This would create a huge moral dilemma within the League.
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.
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.
Batman's allies such as Commissioner Gordon could be involved. Sometimes Batman might be angry at Gordon if he feels that the police force is not doing enough to support his fight against crime. Also, other members of the Bat - family like Batgirl could be part of the story. Their actions or decisions might trigger Batman's anger. For instance, if Batgirl decides to take on a powerful villain alone against Batman's advice.
Obviously, Batman/Bruce Wayne himself is at the center. Alfred Pennyworth would be very involved as he has known Batman's identity all along and would likely be trying to protect it or deal with the consequences if it's revealed. Commissioner Gordon might also play a big role. He has a close relationship with Batman and would be affected by the revelation. Then there are the villains. The Joker could be one of the main villains in such a story, using the revelation to his advantage in some devious way.