Originally Posted by Schlosser85
We don't have any way of having any idea what he thinks about Mr. Freeze, Penguin, or Poison Ivy, and I think it's overanalyzing his comment a little, considering he was only talking about a villain to follow The Joker who wasn't too similar to him.
I think it is safe to assume Nolan is speaking about villains that fit in with his version of Gotham. Mostly the "mundane" rogues like Riddler, Penguin, Black Mask and any others I missed that aren't supernatural or overly sci-fi like Clayface, Freeze and Ivy.
Or perhaps he couldn't think of a way to present the Riddler or some of these other rogues without copying Joker (to a greater extent than he did with Ra's and Bane and Talia).