Buried in Slash Film's Batman 3 round up is a telling contribution from screenwriters David Goyer and Jonah Nolan.
Film asked them which villains they were currently considering for a possible Dark Knight follow-up. “We’re not gonna tell you… other than to say that Batman has been published for 70 years. In the first movie we use Ra’s Al Ghul and The Scarecrow, who had not been in the movies before, and had not been in the sixties TV show before. And there are dozens if not hundreds of other characters that fit that bill. Everyone says its gotta be The Penguin or Catwoman… well I completely disagree.”
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Source: (Slash Film)