Personally I never got into the anime, so I'm not familiar with the character but I will say for arguments sake that it would be nice to see Hollywood show some diversity when it comes to casting if Margot will be playing the lead character. To the casual movie going public they would need to court she isn't well known and isn't moving tickets off her name alone. My thought is she got the opportunity with Wolf of Wall Street, so why not give an opportunity to someone who might not be that well known so they could potentially blow up with a big role this? If the movie is a success it will largely be based on the promotion, marketing, selling it as a cool sci-fi action flick first and foremost. If the thinking is always we need a big name (i.e. someone white) or else not enough people will go see movie x, the cycle will never change. I know it is largely a mentality that the people in Hollywood reinforce, but I think the casual film audience isn't as concerned as they think. Things don't work in Hollywood, until they do and suddenly every studio is trying to copy a winning format.
With the increase of importance of international audiences, particularly in China and other countries this could have done well overseas with Asian countries with an Asian actress. Transformers 4 did major bank in China, partly because they cast some big name Chinese actresses in it.
It didn't have to be an actress who is strictly Japanese either, most Americans can't tell the difference anyway
. Hell they could even have got someone who is half Asian or at least looks Asian. Not knowing anything about the character or what it would require, I'll toss off some names.
Maggie Q
Jamie Chung
Both have martial arts backgrounds
Moon Bloodgood - she at least looks part asian, plus does action on falling skies
Grace Park
Julie Estelle - played Hammer Girl in The Raid 2, just tossing some people out
Having said all that, Margot is hella fine and it is crazy how much she resembles Jamie Pressley