I kind of want a blend. The new underworld king who has ruthlessly taken over is the Black Mask, a mysterious figurehead. As it turns out, Black Mask is Roman Sionis, the new golden boy of Gotham.
Black Mask and his crew could be at war with Rupert Thorne. Maroni could be an enforcer on either side, preferrably Black Mask's.
Alberto Falcone could also be in the mix, as he has been recruited by, let's say Thorne, for information or something. Thorne regards him as weak, but doesn't realize how psychopathic Falcone is.
Oh, and I think the Oswald Cobblepot (The Penguin) should be a part of the underworld, a British gun-runner.
Roman Sionis / Black Mask - Jake Gyllenhaal (Jarhead, Brokeback Mountain)
Sal Maroni - Gabriel Byrne (The Usual Suspects, Assault on Precinct 13)
Oswald Cobblepot / Penguin - Timothy Spall (The Last Samurai, The Prisoner of Azkaban)
Rupert Thorne - Albert Finney (Big Fish, Erin Brockovich)
Alberto Falcone - Matthew Goode (Match Point, Chasing Liberty)
or
Joseph Gordon-Levitt (Brick, Manic)