The Penguin (too 'tricky' to do realistically)
It still puzzles me that Nolan thinks this. The Penguin is one of the easiest Batman villains to do realistically. He's simply a short, big-nosed thief or gangster who likes fancy dress and has pretentions of being a sophisticate. Easy enough to ditch the bird fetish and umbrella gimmicks and just have Oswald Cobblepot as a mob kingpin who's called "the Penguin" (not to his face) because of his squat build and penchant for tuxedoes.