I loved this film, it was all I wanted it to be.
The best dissappearing pen trick of all-time.
Here are some of my thoughts and theories about a next movie;
While I thought Ledger was absolutley brilliant, I think it would be a shame to dismiss the Joker from this series when the character has soo much potential, especially with the possibilty of a Harley-Quinn relationship developing now he's in Arkham... it would be hard to replace him, but I think it needs to be considered.
I'd also like to see Poison Ivy make an appearance, but not in a traditional way. I think it would be a good idea to just have her as a powerful and manipulative love interest of bruce wayne, sort of like her portrayal in the long halloween comic series where she infects and clouds his judgement, however without her power to control plant life. Charlise Theron Please!
It seemed to me like the ending of this movie really leads to the idea of an antagonist seeking batman's true identity, which definently gives the chance of the riddler surfacing, maybe just as a private detector like he has been portrayed lately in comics (So I've been told). Leo Dicaprio would be great as many of you have mentioned. Perhaps some darkhorse possibilities could be Edward Norton or bob Downey jr.
Whatever happens, I have great faith now in Nolan creating a great ending to a trilogy or whatever he might have in mind!