The Nolan trilogy was pretty self contained. Even if Third Rock suits up as Batman (and I really don't know if I can see JGL as Bruce Wayne) you're re-introducing one of the tentpoles of JL, hell of DC Comics, in half a movie, which really doesn't seem fair to the character or allow any worthwhile character development.

Mind you, I'm not saying there needs to necessarily be a 3rd Batman reboot, like The Amazing Spiderman (though it must be said my intense dislike for the movie has lessened over time). And yes, if you're looking at The Avengers you have a major(ish) character brought on in the ensemble cast. But Batman isn't Hawkeye, and it would seem to be a disservice to bring him in a teamup movie when we have no grasp of what kind of Batman we're working with. Yes we know the backstory, but that didn't stop Miller's Batman being different from Burtons, or Shummachers, or Nolans.