I love the portrayal of Killer Croc in
Arkham Asylum, how in terms of inspiring sheer terror, he seems second only to The Joker amongst Gotham's crazies. Though the look would probably not work in a Nolan film, that murderous, vicious, vengeful persona that came across in the cut scenes and interview tapes could translate into a really compelling villain in other mediums. For starters, it's about time someone told a really good Croc story in the comics. And he might just work out in a film too, if handled right.
I agree with the people saying he needs to look like he does in
Joker: an imposing black man with scaly skin and filed-down razor sharp teeth, with an implication he is a cannibal. But I'd propose sacricing size for screen presence, and casting a powerhouse actor like Forest Whitaker to really bring some scary intensity to the role.
As for what role Croc could play in a film, I imagine that in film number three, Batman's two main foils would be Catwoman and Riddler. But since - again, how I would like to see it - Catwoman is the love interest, and Riddler is a fed/PI on Batman's trail, Batman needs a threat who is an out-and-out criminal. Enter Croc as a new ganglord, a freak aiming to fill the void left by the dismantling of the gangs over the previous two movies.