Though there is some speculation that Ventimiglia's character could eventually become The Joker, that's not who he's starting off as. Warner Bros. TV has has announced that the former Heroes actor will play a seductive serial killer named Jason Lennon, aka “the Ogre.” This is not the established DC Comics character of the same name, but an original creation for the show. Lennon is described as a "killer who has been preying on the young women of Gotham for nearly a decade, luring them into his web and confronting them with a series of “tests” as he searches for his perfect mate. When the women fail to live up to his impossible standard, Lennon disposes of them quickly and viciously." The press release also mentions that the Ogre will have a "major, multiple-episode recurring arc", and that "his obsessions, combined with Gordon’s determination to bring him down, send both Gordon and Lennon — and those around them — on a course toward tragedy." Interesting addition I'm asure you'll agree. Expect Ventimiglia to debut in episode 19 of Gotham.
Source: Warner Bros. TV