Okay, here's how I'd do Croc to make him a substantial character that reflects on aspects of Batman as well as Nolan's take on Gotham. An all-out lizard monster just ain't gonna happen and would obscure what makes Croc work best in the comics. So here goes.
In the comics Killer Croc has always been a bit of a gangster as well as a brute. Before he was mutated he liked to dress nice and have ladies, power, prestige etc and he's actually lead successful crews before. So I'd start off with a very capable and half civilised Croc who loses it all and is left in the sewers, thanks to the jerk Batman.
Imagine a modern pimped-out gangsta boss who just happens to be very ugly with scaly grey skin, huge hands & teeth and a gargantuan build. Give him cornrows, tattoos, a diamond grille for his teeth and polished guns and knives... although he IS partial to ripping up guys with his hands as well. As a child he was a poor little 'freak of nature', a kind yet frightened weakling who was forced to cultivate an fearsome image in order to survive in the worst of Gothams hoods. He was VERY successful at that, taking control of a major Gotham crew and relishing in his fear and respect...however deep down he's still scared of being a monster. Underneath his power and image he's psychologically frail, and because he's actually begun to enjoy ripping people up (rather than just doing it for show) he secretly wonders if the monster he was forced to become has totally devoured and ruined the innocent boy he once was. In reality, the little boy freak never survived at all. So in a sense Croc is a reflection of a young Bruce if he let The Bat, the rage and the insanity consume him.....
..so when Batman confronts Croc and strips away his luxury cars, fur-coats, girls and jewelry, he's left completely bare and exposed and in a sense, inhuman. The trappings of power and luxury was all Croc ever identified with, his grip on humanity, life and success. What used to contain all his rage for being bullied, hungry and crushed for most of his life is now gone, and he's forced back onto the streets where his life began, a savage in the filthy murk of Gotham. A woman walking with her boyfriend sees him dragged into an alley by a snarling grey shape. She sees a scaly, naked man chewing on her beloveds neck, she screams, police arrive, they're slashed across the face and guns stolen, a sewer cap lies upturned and the beast is gone.
Batman realises he's created a worse monster. He tracks him down again and appeals to him, offering help, but Croc begs Batman to leave him alone and let him retreat to the swamps where he won't harm anyone, just wants live and be alone. Batman refuses and Croc almost kills him. Eventually he's beaten and put him away in Arkham, where he can either resume his power and position as the top freak, or just be alone. It's all very sad.
