Batman is Gotham's dark knight. Batman IS Gotham. Gotham City stories are always relatively self contained, even in today's comics. Gotham's criminals are the mafia, the bank robbers, the gangs, and the human psychopaths. Generally not so many super-humans; they stick to other cities or never rise up because Batman takes them down. But TDK is still in Batman's early years- the super-humans are what comes after. There's a chance we could see some like Mr Freeze or Ivy, but we probably won't, because Nolan's Batman is intended to be mature and realistic, and for now it's just difficult to fit them in appropriately; but more importantly kind of redundant, because there are still human villains to be used- Catwoman, Harley Quinn, Riddler, Hush etc... Hopefully next we get two of those. Selina Kyle is Selina Kyle, and works well with the right writing and casting. Same with Harley- one anon made an interesting post about her being a prostitute and the Joker was one of her clients, etc. etc. etc... Riddler, well... Tone down the Carrey thing a LOT, even if it gets to being just a guy in a suit with a question mark tie and cane, and focus on mystery.
As for Two-Face...
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So... Yeah...

But it's kind of an accident. Batman takes the blame for Dent's kills and Dent's own death.[/hide]