Huh. Whaddaya know.
It's utterly-impossible to "predict" who or what he'll be playing, because that's not how Nolan and company roll: He could be playing anyone, re-imagined into anything, and it'll probably work out (and if not we'll spend years pretending that it did anyway) because Nolan Can Do No Wrong.
In any case, might as well make a game of it: Who can come up with the most preposterous rumor as to who he's playing?? I'll start:
He's Dick Grayson... but NOT Robin!
He's Clark Kent... but NOT Superman!
He's Bane... BEFORE the experiments!
He's Jack Ryder, alias "The Creeper" (short version: What if Keith Olbermann was secretly The Joker, and a good guy.)
He's Harley Quinn - Nolanverse Joker is even more "reimagined" than we thought!
The Joker: 2.0!
Azrael! (good god, I hope not...)
The new Ras Al Guhl!
Vic Sage, alias "The Question!"
The Riddler - Turns out the Nolan's really, really liked your photoshopped fan-poster!
The Ventriloquist! (I love that guy)
Superboy Prime - he suckered-punches reality, thus explaining why the various DC movie heroes don't live in the same universe now!
Surprise! The vaugely Robin Hood-esque bandit Alfred supposedly killed back in the day had a son! And he likes to hire Luchadores and kitty-cosplayers to fuck with his enemies!
Clayface. Why the hell not?