Even though the casting choices where an issue, I don't really think that was the primary reason the movie bombed so hard. The characters were simply flaccid, the plot uninspired and the bending-visuals sluggish and crude.
I know it's a series strongly inspired by Eastern cultures, but it's still a fictional series set in a fictional world. So how the characters have to look in a life-action adaptation is open for debate because you can't directly translate the cartoon characters into a real world ethnicity.
What you could however is keeping the integrity of the characters intact, which was far from the case in the movie. Zuko's complexity was non-existent, Sokka had no sense of humor, Katara was a shadow of her strong-willed animated counterpart and everyone's favorite grandpa Iroh was a farce. I'm getting all cringe just thinking about the scenes where he gives Aang a rock to see if he's the Avatar and where the firebenders run away when they see he could make tiny flames on his hands -.-