I know that tonally it would be all wrong, but it would be amazing if at the end of Man of Steel
(post credits or whatever) Christian Bale is sitting in Clark's apartment with a "proposition" about forming a group of heroes.
And while tonally and logistically it would be difficult to manage effectively it could work. The Bruce Wayne of the Nolanverse is more willing to play with others now and share his toys, he is somewhat distanced from Gotham(maybe now he cares about the whole world etc). He is in a good place to be sort of the founding father of the Justice League.
Will it happen like that? Probably not, but it would be sweet if it did.