Who are you looking at? You're looking at the person that is *quote-unquote* "Katara"
Also, for Dev, he doesn't look too bad. Just shave off the head and tie it into a ponytail, make the scar more visible and "scar-like" and he'd be a good Zuko.
Now the only real problem are the soldiers. "White" Fire Nation Soldiers won't do. Heck, even the eskimos are white.
I now predict the movie will be praised for its fight scenes and possibly the story, but it will ultimately fall down and fail when it's about the "almost all-white" casts