I seriously doubt we'll see The Riddler in the next film. The character is just far too similar to the Joker.
What I loved about this movie more than anything...is the fact that is really isn't a "comic book" movie. It can better be described as a classic crime drama along the lines of Heat. THe film is a tightly woven allegory concerning moral ambiguity and how one deals with severe trauma.
The relationship between The Joker and Bruce Wayne (I like to call him Wayne rather than Batman as Batman really wasn't in the film that much) is so brilliant told. They are like the two sides of the same coin in (yet another tie in to the Harvey Dent character as well) in terms of how someone deals with crisis and tragedy. It's obvious The Joker suffered at some point in his past...as did Bruce Wayne...the difference coming in how the two deal with their pain. The Joker becoming an instrument of chaos and The Batman becoming a spirt of order.
This relationship is so perfectly shown in the interrogation scene. The Batman asks The Joker why he wants to kill him to which The Joker responds that he has no desire to kill him...he says they complete one another...that they are "destine to do this dance forever"...towards the end of the film when The Batman has The Joker tied and hanging upside down...they carry on yet another moment of brilliant dialogue punched up by the imagery of of The Joker being upside down and The Batman being upright...again playing on the "two sides of the same coin" allegory...
Add to all of this Harvey Dent...who is used to represent the turmoil that twists inside both Bruce Wayne/The Batman and The Joker...as The Joker mentions..."anyone can do what I do"...with a push in the right direction...this smartly comes to a head when Dent...who has represented everything The Batman fights for is "pushed" by The Joker and proves his point...
...I could go on and on...needless to say this was one of the smartest films of 2008 on top of just being a great movie!