If it was me, i could make it beat Titanic, only because i know story structor and some character history... some being that i would be able to tell a story without being too bogged down by the characters complete history...Which these two things made TDK good...
The key to this sequel is simply that it needs to be made if the right story is there, if ther right story isnt there then hold fire.
See for me, if i was going to make the next movie big, i would have to explain what the joker ment by a better quality of villian... I think The Joker set the mark, so we need to be extreme in the next movie and i think with characters such like Black Mask, Catwoman, Penguin, Riddler, -- There all expected to appear. Me personally believes that because someone expects it, the anticipation is set high and the disappointment level happens because you set The bar to high.
So i would chose Mr.Freeze - In Mr Freeze, i could forsee an epic story for a number of reasons. Firstly with Mr.Freeze - we couldnt simply follow his traditional man fell in ice and now wears a containment suit... It wouldnt fit into the nolan universe of batman... But doesnt mean we have an interesting character...
I see The Dark Knight as a bit of a Cause and Effect kind of movie, best demostrated by The Joker and Two-Face... The Joker set about giving gotham a better class of villian. He turned a good person into a villian. Two-Face. Well i see something similar occuring the Victor Fries...
What i would like to see would be to create a cause that effected several hundreds of people in Gotham. For that i would say we need something like the collapse of Wayne Enterprises... what would happen to Bruce? Lucious Fox? Alfred? see if Wayne enterprises was taken over or closed down, any funding in research would be stopped... See where a character like Victor Fries would be effected by this cause, He holds bruce wayne responsible for his research, which means that the villian of the film isnt targeting Batman...but Bruce himself... how can Bruce and Batman appear at the same time to of which we would need to see Batman's apprentice... i think it would be too obvious to have robin. But i like the idea of an apprentice, if i was to introduce a character to the universe as a sidekick to batman, i would do it via The Riddler... Edward Nashton would initally be an employee who discovers who Bruce Wayne really is and instead of going to the press, mostly out of fear of what The Joker would do if his identity was revealed. Edward joins Bruce to rebilled and be apart of the whole Batman saga. However obvously Edward would then at some point become the Riddler and breaking Batman;s trust which would make you understand why he is so avaisive to want Robin join him in the future. Because Edward a previous assistant was consumed by his ability that he didnt realise it made him a villian.