That said, it seems unlikely that he'll have the flexibility, while working on directing a massive undertaking like Justice League, to take on a solo film, so the in-development movies like Aquaman are probably out of the question.
Who could Affleck play, then, if he were going to have an appreciable role in Justice League while still keeping his schedule open?
Depending on the version of the League, there are a number of reasons why Max Lord might be needed as a backer, liaison or whatever other role he could be placed in. Setting the Justice League in a somewhat more real-world setting, as Warner has done with Superman in Man of Steel, would require a concession to the reality that the U.S. and global governments likely would not allow a group this powerful to exist without oversight of some kind, and Max would be the obvious name, since he's the one with the longest, richest backstory and some great potential for future stories.
The only problem might be that if they're developing a Booster Gold series at Syfy, it's hard to imagine taking Max off the table for that show. And even if the Booster show were to be in the movie's continuity, there's just no way you'll see Affleck slumming it on Syfy.
Kyle Rayner (or Guy Gardner)
Affleck, with a little hair dye, could make a terrific Guy Gardner, but honestly if they were going to make him a Green Lantern, my money's on Kyle Rayner just for the physical resemblance.
In any case, I could see Affleck casting himself as Green Lantern. Following on the heels of a Ryan Reynolds-led movie that was as poorly-received as any solo superhero film arguably since Daredevil, Affleck might think that the Green Lantern franchise has nowhere to go but up and that if he cast himself in the role of "another Green Lantern," there are two possibilities: the audience doesn't love him, but it doesn't matter because you can always go back to Hal, or move on, for the sequels...or the audience loves him and he's established in a franchise.
The argument against? It seems likely that John Stewart will be the Green Lantern of choice for a Justice League movie; not only is he known as "the Justice League's Green Lantern" from the cartoon series, but they'll want some ethnic diversity on the team just in general.
To have Ray in an advisory capacity, as he currently is in Frankenstein: Agent of S.H.A.D.E., Affleck could still be a hero while avoiding the dreaded tights that would draw unwelcome Daredevil comparisons. It would also give Affleck an opportunity to play against type as a smart guy, and prove with the whole world watching that he can act if he's got a good script.
It also, of course, sets up the possibility of an Identity Crisis story in the future. While that might put some fans on edge, it's the kind of story that, like Marvel's Civil War, can't be completely ignored when you're trying to think of what might be a marketable film for a mass audience.
Why not him? Well, let's be honest--guys like The Atom and Firestorm are probably going to be held in reserve for the second movie, if they ever appear on the big screen at all.
J'Onn J'Onzz, Martian Manhunter
This one could be a clever way of Affleck avoiding a franchise if he didn't want to return. Play a shapeshifter! Not only is Martian Manhunter unlikely to ever have a solo film of his own (he could hardly ever support a solo comic!), but anyone can play him from movie to movie and provided they handle the transition correctly it wouldn't be an issue.
You'd also have the benefit of probably being able to pawn most of the inevitable motion-capture work off on someone else, leaving Affleck to do only the very few scenes of John Jones, detective. On top of that, nobody has a real sense for what J'Onn's human side looks like, so Affleck wouldn't be alienating fans.
I can't even think of a good reason why this wouldn't work. J'Onn is my odds-on favorite for if Affleck had to take a role in this film.
You laugh, but it could happen. Affleck has been either the lead or A lead in every movie he's directed. It could happen.
No, it really couldn't.
Why not? Well, Daredevil, for one. And also, he probably doesn't want to get roped into a long-term franchise contract for movies he likely won't direct.