8. The Penguin
Whilst Christopher Nolan has publicly voiced his dislike of the character numerous times, claiming he would never include him in his series, he said the same thing about Robin as well, even if he wasn't technically Robin during The Dark Knight Rises. The Penguin could, like Black Mask, have easily taken the place of a mob boss within the series, as well as a criminal mastermind, running the whole mob with a precision and ferocity that was unseen from the likes of Falcone or Maroni. One interesting fact about the Penguin that distances him from other villains in the Batman universe is that he harbours no psychotic obsession, instead working his criminal empire wit intelligence and the grace of his aristocratic personality.
Although Nolan rooted his world in reality, there is no reason that the Penguin could not have just been a normal man, with highly webbed toes and fingers, and he could have maintained his obsession with birds, although it is almost certain they would have trashed the attack penguins that were popularised when Danny DeVito played the character in Batman Returns. The Penguin's array of umbrellas could definitely have worked in a realistic Gotham City as well, with them containing guns or other weapons for him to use as necessary.