Batman Begins took a lot of elements from Year One and The Dark Knight took a lot of elements from The Long Halloween and it only makes sense for the third Batman movie to take elements from Dark Victory.
Now, for BB, the villains, or the main villain needed to be apart of Batman's origin, i.e. Ra's Al Ghul, and then for the big villain that is based on Gotham, it was Scarecrow.
TDK and TLH had a lot of same elements; Joker, Two-Face and dealt more within the mob trying to control Gotham.
Now Sal Maroni SHOULD still be alive, and he was in DV, so I think one villain that could be much like Hangman would be the Phantasm. A female villain would be perfect for the third movie. The third movie needs at least one female cast, and also as a love interest for Batman, but having a villain kill members of the mob would make Batman come out of the shadows even with the police after him.
Just my opinions.
But a second villain to have could be Black Mask, not as a mass-murdering villain, but as a mob boss, trying to figure out who the Phantasm is. And a cool Batman/Phantasm/Black Mask fight would be nice to see and something very different.