Just to start things off, this article is based purely on speculation and rumor, so it could be entirely in vain, or the official casting announcement could happen and it would still be in vain.
Yesterday the internet lit up when Deadline reported that Joseph Gordon-Levitt was offered a role in The Dark Knight Rises, and of course speculation is now rampart over who he could be playing. Ever since Inception JGL's name has been attached to the next Batman movie, a fan favorite to play The Riddler. The rumor mill seemed to die down when Levitt's Inception co-star Tom Hardy landed a role in TDKR,but once again speculation and rumors are in full swing.
If the rumors are true then I'm not surprised; Nolan seems to be very loyal to his actors and it was clear from Inception that both Hardy and Gordon-Levitt were the break out stars of the film. I took to the message boards last night to see want fans were predicting, and there were some far out theories over Gordon-Levitt's casting to say the least.
The Riddler rumor popped up almost immediately, personally I hope this isn't true simply because I've never been a fan of The Riddler, he always just seemed like a Joker knock-off to me. Nolan himself has said that Riddler isn't a villain in this film though so that rules him out.
By far, the most out there rumor I've read is that Joseph Gordon-Levitt is playing the Joker. Yes, JGL does resemble Heath Ledger, but why can't we let the Joker rumors die? I've heard everything from an all mo-cap Joker to "unused footage" from The Dark Knight as a way to bring the character back. Now I love the Joker, I would love to see him return to pester Batman, but Nolan himself has said that the character will not be returning. It seems like Nolan sees it as disrespectful to Heath Ledger to recast the role (or create an all CGI Joker) and I tend to agree. Just leave Heath's portrayal as is, bringing the Joker back for TDKR in any capacity just seems like it would come off as a cheap way to cash in on the popularity of the character. Now do I think we've seen the last of the Joker? Not at all, at some point in the future Warner Brothers will look for a way to make some quick cash and they'll look to one of their most profitable franchises to do it and we'll get another Batman reboot with another director and cast (we may not want it to happen, but it will), but until that day comes just let the Joker be.
But the most interesting rumor out there is that Joseph Gordon Levitt will be playing Dick Grayson. Both Christian Bale and Christopher Nolan have gone on record about their dislike for the character of Robin,but just because it's Dick Grayson, doesn't mean it has to be Robin. Many people write off the character of Robin as the "plucky sidekick", mostly due to bad memories of Burt Ward's catchphrase spouting performance and Chris O'Donnell whining up a storm. But in the best Robin stories he serves as a conscious and the human side of Batman, often keeping Bats from falling too deep into the darkness.
The Dark Knight ended with hope for Gotham City, but Batman in a very dark place, losing the woman he loved and on the run from the police for murders he didn't commit. When we pick up with Bats in TDKR he can be much darker and brooding than we've seen him before and taking a far more brutal stand against crime. Enter Dick Grayson, a young man who lost his family to crime and uses Batman as his personal inspiration, using the persona as Nightwing to protect Gotham. At first Batman opposes Nightwing, thinking him as another copycat, but then Bats is almost killed by Bane and Nightwing comes in to help, narrowly saving Bats and himself. A wounded Batman could see a little bit of himself in Grayson and start training him as his partner. This would also continue the theme of Gotham City's continuing redemption, showing Grayson as the first of many people that want to take a stand for their city, but uphold the values that Batman stands for(i.e no guns or killing).
If you watch Inception, you can see that,Arthur, the character Joseph Gordon-Levitt plays, isn't too far off from the character of Nightwing. He serves as right hand man to Cobb, Leo's character, but also isn't afraid to speak out against him and serve as voice of reason. We see that Arthur, is still inexperienced and seems to be learning from Cobb. But though he's inexperienced, we also sense that Arthur thinks he is much more adjusted and more normal than Cobb, as well as less obsessive. Now if that doesn't sound like Nightwing and Batman, I don't know what does.
But then again Nightwing could be totally off too, just look at the recent reveal of Tom Hardy being Bane. Fans were so busy chasing any rumors they could find (usually Hugo Strange ones) that we all seemed to forget about Bane, so when the announcement came it came as a genuine shock. Gordon-Levitt could be Black Mask as well, a character who could fit in quite nicely seeing how he has a history with both Catwoman and Bruce Wayne. There's a multitude of other characters JGL could be playing, if anyone. But in typical Nolan fashion, we probably won't know until he wants us to know.