Hmmm. Maybe it's because I don't really know Captain America other than as an ironic nickname for other characters, so I can see Will Smith in that role no problem. Superman is and always will be Christopher Reeve for me, but that's mostly because they shot it in my home town, so I have a special attachment to those movies.

I think playing around with the cultural context of a story is fine, if you change the whole thing. I haven't seen Tortilla Soup, but it sounds cool. Thing is, then it becomes an examination of that storyline from a Mexican American cultural perspective. If they had tried to still set it in Chinese culture but with a Hispanic cast, then it would all go horribly wrong.

While sometimes the Wonderbread casting of popular culture makes it difficult at times to distinguish between pop culture and 'white' culture, White people of various nationalities do have legitimate cultures that are not interchangeable with any other.

If you cast an Asian Emma, you'd have to change the other elements of the cast and setting as well, so that it would become an entirely different interpretation of the story. You couldn't pretend that it made no difference. But if you do change enough other elements, you could have an Asian version of Superman, I think.