Ignore race completely.
No, I'm entirely serious. If someone is best for the role, chuck them in. Remove all racial homogenity (except for within families, and even then have some leeway if you the parents are different 'races), and emphasise more heavily the cultural differences between the nations.
It'd get rid of half the casting problems (people being cast for roles they only look like and are awful for), the other half of the casting problems (actors who are deliberately cast to look nothing like the source character, and are still awful).
You'd get multiple other advantages too>Disguise episodes aren't entirely unrealistic>The bloodbender now has an excuse to not have been lynched the first instance someone went missing>That's right motherfucker, I know how the countryside used to work>You can't rant against it, because that'd be racist>You can't rant for it, because that'd be culturally disrespectful>Everyone can just shut up, and look as we watch why ATLA should not be a live action adaptation>In the case that it is succesful, they can't just brush Kuvira's ethnic gulags under the bed, and she will be acknowledged as a nutter worse than everyone except Ozai.>Seriously, what the fuck. 'I'm an orphan who got adopted by the richest people in the richest city state, and spent half my life as a spoiled pampered dancer, this was justified'. Go neck yourself, you moley cunt