That doesn’t give us much to go on in terms of who he’ll be playing, but that’s never stopped anyone on the internet before. Dafoe is a reasonably famous guy, so he’s probably not playing a nobody (and that would make it impossible to guess anyway), which means he’ll most likely be a recognizable character from the comics. Most of the supporting cast for the main heroes is already filled in thanks to Man Of Steel and Batman V Superman, so we know he’s not playing a guy like Alfred or Perry White. He could be one of the superheroes, but he’s not really young or sexy enough to fit in with the current group unless they paint him green and make him The Martian Manhunter—though screenwriter David S. Goyer would probably never allow that to happen.
That leaves some of the slightly more boring behind-the-scenes movers and shakers of the DC Universe, like Maxwell Lord or General Sam Lane. Of course, both of those characters are only sort of “good guys” in the comics, so they could also be wrong. Then there’s the possibility that the “good guy” thing is just a smokescreen, and he’s actually playing a villain or a bad guy pretending to be a good guy. If that’s the case, he’ll just end up getting killed off when Superman snaps his neck or when Batman blows up a car he’s driving.
Justice League: Part One is set to be released in theaters in November 2017.