Green Lantern? Hell, Ryan Reynolds is also gonna play Flash, Batman, Superman, Aquamn and Hawkman! 
I would think they would want a few more superhero movies off the ground before Justice League, rather than introducing them in the movie.
Though if there's no new Batman before JL and Nolan's Superman doesn't fit into it either (and if the Wonder Woman TV show is a success), how about casting older actors as the big three who are putting together the league? That way no idiots will confuse them for Bale's, Cavill's and Palicki's versions. You could bring in John Hamm to play Superman. Catherine Zeta-Jones as Wonder Woman? Who could play an older Batman? Chris Noth? Tim Daly? Russell Crowe? Alec Baldwin? Dennis Quaid? Kurt Russell?

I would think they would want a few more superhero movies off the ground before Justice League, rather than introducing them in the movie.
Though if there's no new Batman before JL and Nolan's Superman doesn't fit into it either (and if the Wonder Woman TV show is a success), how about casting older actors as the big three who are putting together the league? That way no idiots will confuse them for Bale's, Cavill's and Palicki's versions. You could bring in John Hamm to play Superman. Catherine Zeta-Jones as Wonder Woman? Who could play an older Batman? Chris Noth? Tim Daly? Russell Crowe? Alec Baldwin? Dennis Quaid? Kurt Russell?