Well, Batman's Rogue Gallery of realistic characters isn't that vast. The main ones are The Riddler, Joker, Two Face, ScareCrow, Rhas Al Ghul, Black Mask, Red Hood, Zsasz, Mad Hatter, Hugo Strange, Hush, The Penguin, Calendar Man, Bane, Deadshot and The Ventriloquist.
Riddler, Joker, Two-Face, ScareCros and Rhas are out.
Bane, The Ventriloquist, The Penguin, and Mad Hatter wouldn't really fit in the Nolan-verse.
Zsasz and DeadShot just regular serial killers, which wouldn't make for interesting subjects in a two and a half hour Batman movie.
Calendar Man would actually be really interesting if it weren't for his stupid name.
Red Hood, depending on his incarnation, is either Joker or Robin, and both are out of the game.
So to me, that leaves Black Mask, Hugo Strange and Hush.
So I'm rooting for the sadistic son of a **** that kidnapped CatWoman's sister and made her eat her husband's eyeballs.
This is exactly why I want him in.
Also, I forgot about Hush, he'd be a good candidate, but it may hinder some of the plot by delving into his backstory, which is very necessary.
Plus Black Mask would make the most sense. With Gotham's crime lords all dead thanks to the Joker, now's the perfect time for a Mafia Overlord like him to step in and unite all the gangs of Gotham under his rule.
That's exactly my reasoning.