AU where the Justice League forms like usual, except Batman maintained his “totally a myth” status and has in fact been active for years before the JL forms. He’s very cautious about trusting them, but still joins, and the others sort of accepts that as long as they trust that Batman has a really hard time with trust, it will all work out in its own weird way
Then, one day, in the middle of a JL mission, the League gets in a tight spot. Out of nowhere, this blue and black blur swoops in and saves everyone’s ass. Maybe breaking some shackles that were proving very difficult, maybe disarm a bomb that the League was just a hair’s breadth too slow to reach without help, but whatever happens, the shadowy figure pauses just long enough to say, “Hey, Batman, you know you there are these things called cellphones now and you can just call sometimes, it doesn’t have to be this dramatic?” and bounds away after shouting ‘let’s do brunch! Bring your new friends!’
Batman is mortified.
No one lets it go.
The entire rest of the mission, the whole League is asking so many questions. Who was that? Do you know him? How do you know him? What’s going on? I didn’t know there was a vigilante in this area?? They don’t let up until he talks.
“That was Nightwing.” Batman is mumbling. The JL forces him to bring them to the Brunch. Brunch happens to be in a run-down apartment on the edge of a bad neighborhood, at five in the morning, in costume. Nightwing introduces himself as Batman’s lovechild with justice.
“I did not realize Batman had a child,” Martian Manhunter says, calmly enough that no one’s sure if he’s accidentally plucking a really loud thought out of the air or if he’s trying to make a joke.
Nightwing stares for a moment falling over laughing. He doesn’t get up. Batman starts trying to apply anti-Joker venom but Nightwing just kicks him and laughs until he cries. He keeps trying to wipe his eyes and his mask keeps getting in the way, so he asks everyone to leave so he can please get a hold of himself
He is still laughing when they leave. Everyone is confused. Batman is furious. Nightwing manages to breathe long enough to say, “We’re just so glad you’re socializing now, Batman.”
Superman turns to look at Batman very slowly. “…’we’?”