Warner Bros loves Ben Affleck. The Town was great for them, and Argo looks pretty promising. They want to be in the Ben Affleck business so much they're just throwing projects at him. Big projects. The previous one was a two-film adaptation of Stephen King's The Stand. The latest is Justice League.
According to Variety Affleck is the only director who has been sent Will Beall's script for the film. The trade speculates that Affleck only directs movies in which he stars (errr, Gone Baby Gone?), so that he might play a character in the film. I sort of doubt it. I also doubt he'll do the movie.
He shouldn't do the movie. Gone Baby Gone, The Town, Argo - these are the sorts of mid-level, adult movies that rarely get made anymore. We need more films like this. Justice League will work itself out*, and Affleck is needed in the realm of smart, tight, grown-up movies. I'd accept The Stand, but Justice League? Get the fuck out of here with that. Anyway, Argo features Jack Kirby - that's better than any other comic book movie possible.
Like I said, I doubt this sticks. Affleck hopefully knows he's in the Sidney Lumet business, not the James Cameron business.
* ie, I suspect they'll blow it.