Christian Bale in "Justice League": How they could do it

While there are no immediate plans to make a Justice League film in the near future, I think that if carefully explained, other superheroes existing in the Nolanverse would work. Christian Bale was a great Batman, and the general public will continue to see him as the Dark Knight for years to come, so why should they replace him? If he is alive, as the ambiguous last scene would lead us to believe, then there isn't really a crucial reason why he couldn't return as Batman. What they could explain in the Justice League film is that Bruce Wayne discovered the existence of metahumans and alien lifeforms while travelling the World, but because it was such a well kept secret to the World at large, he kept this fact to himself. Over the years, he would investigate unnatural occurances, such as a meteor landing in Kansas. Wayne eventually discovered the existence of kryptonite, but he didn't know what it was or exactly what significance it had, so he kept it but promised himself to study it when more information was at his disposal. After he left the mantle as the Bat, that's when Superman, Green Lantern, and the Flash would emerge. He would find these heroes intriguing, but found their ideals and methods far too different from his own. Eventually he would return to Gotham after it was attacked by none other then Darksied's Alien army (if they use Darksied) and he would accept that the World needed Batman once again (Yeah, there's still the idea of JGL's character, but they don't NEED to make him Batman, do they?) He would eventually meet Superman and find common ground between the hero, and eventually call upon the rest of the World's metahumans and finish Darksied off. That's one way they could do it. I hate the idea of them choosing a different actor and I hate the idea even more of Joseph Gordon Levitt starring in the film.