A:TLA had Caucasian fans because many identify the characters as Caucasians, especially with the white voice actors. Heck, some fans even thought Toph is European! o.0 The anime style is like a Rorschach inkblot test - you see what you want to see. For live-action, it's harder to be racially ambigious. The best thing to do is decide upon the race of nations based to the actors that you have chosen, which is what M.Night had done.
If you think a live-action all-Asian movie is a great sell, then Asian and American studios must have been pretty lousy with marketing those movies, like Red Cliff and The Forbidden Kingdom. If you can't sell all-Asian "big movies" already IN THE CAN, why even finance one? Paramount is a for-profit business, not an affirmative action program.