The success of The Dark Knight guarantees another film - not that they were counting on success to make a third film happen. As far back as Batman Begins it was already confirmed that Christian Bale, Michael Cain, Gary Oldman and Morgan Freeman were all signed on for three films.
Live News is reporting in Batman buzz: Johnny Depp as The Riddler? that Johnny Depp has been approached to play The Riddler and Philip Seymour Hoffman is being considered to play The Penguin.
The story is questionable. On the one hand Johnny Depp would certainly be an exciting choice for any male villain in the Batman universe but on the other hand, mention of the Penguin suggests the source isn't reliable.
I've read direct quotes from Christopher Nolan stating that the Penguin isn't a character he's interested in. In fact Live News in this story, Jolie the perfect Catwoman, says original Catwoman, Christopher's quotes on the Penguin are, well, quoted as follows:
“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,” Nolan told Totalfilm.As much as I would like to see Catwomen in the next film hopefully that quote will kill any chance of Angelina Jolie getting the part (sorry, I just think she's too obvious a choice and doesn't have the range to bring anything new to the role that she hasn't done in other movies). I have no suggestions for Catwoman but I'd be looking for someone who has done gutsy roles and can handle the emotional baggage as well.
Back to Depp and the Riddler. I've heard it speculated that the Riddler might be a good choice as a Joker replacement (assuming the Joker was part of the third film - very likely since he didn't die in the second). If that's the case then my own thoughts are that the Joker should simply be re-cast. I think Depp could certainly fill Heath's shoes.
Heath was outstanding however, no matter how much people think no one can touch his performance, the Joker is Batman's number one arch rival. People will want to see the Joker again. Someone is going to have to play that role.
Depp is a great character actor and he puts the same amount of thought into each role as Heath did with his Joker. I think Depp would be smart enough to not only take on Heath's interpretation of the Joker but could build on it with his own ideas. Depp is 'fearless' like Heath.
All that aside, Nolan's quote reveals that another Batman movie may go for lesser known villains as he did in the first film. It would be a big gamble. The first film was an origin story, thus you'd expect lesser known villains. If the third movie doesn't have a popular villain then the story will need to be even stronger to compensate. A tough ask to pull off for a third film but Nolan's team are clearly up to it.
Nolan has also said he's not interested in bringing Robin into the series. I'm someone who wants to see Robin given the Nolan treatment. It's a difficult character to bring in but I tend to agree with this essay on the Batman on Film web site, OPINION: "Robin - To Be or Not To Be, the timing is right.
Batman lost the love of his life, Rachel Dawes, in The Dark Knight and the film ends with him being pursued as a murdering vigilante. Batman is at his lowest and could use something to help bring him back. Robin could be a good beacon of hope.
I haven't seen a good live action Robin since the 1940's cinematic series. In that Robin was played seriously, almost as an equal and was a very capable sidekick. I'd like to see Robin brought in by the very capable Nolan. It would certainly make up for the lack of a major villain. Here's hoping but I'm not holding my breath.
Getting back to the rumors...
I can't see the Riddler in the third movie because he's too similar to the Joker (at least if you go by previous, live action interpretations). I'm predicting it probably won't happen.
As much as people would like to see Catwoman, she's almost as problematic as Robin to bring into this new, more realistic Batman universe. Again, probably won't happen.
The Penguin I can see in Nolan's world (so long as you erase from your mind Tim Burton's origin story of being raised by real penguins) but he's hardly a 'super villain'. Seems more like a high society, rip off merchant side story than a major force in crime. I've read that Nolan thinks the Penguin is too campy for his universe so there's a good chance he won't be included.
I'm sure there are many more rumors out there. Surprisingly no one's talking Mr Freeze or Poison Ivy (I'm not serious - let's hope it stays that way). I could be wrong on all my conclusions.
What I do know is that it's too early to know anything about a third film for sure. I'll be looking forward to more rumors.