What about Beorn? He could be any ethnicity.
the people of Lake Town will look similar to the people of Rohan who were described thusly:
"[The people of Rohan] have long been friends of the people of Gondor, though they are not akin to them. It was in forgotten years long ago that Eorl the Young brought them out of the North, and their kinship is rather with the Bardings of Dale and the Beornings of the Wood, among whom may still be seen many men tall and fair, as are the Riders of Rohan."
^ From
The Two Towers.
Technically true, but:
1) The passage you refer to is a very minor reference from a different book. Nothing requires the movie to use it as a source for casting.
2) Even if they do, AFAIK, "Beorning" would refer to either the people that Beorn governed after the events of "The Hobbit", or to his descendants. Either might be tall and fair depending on the heritage of the other parties involved, but that doesn't say much about Beorn himself.
3) More significantly, Beorn himself is NOT fair-haired. He's specifically described as having black hair and a black beard.
4) Most importantly, Beorn is a solitary figure at the time of "The Hobbit". There are no others of his people around-- whoever they were. It's speculated that he's the last member of a race of shapeshifters from the mountains. So why couldn't his people have been of different ethnicity?
Seems like this would be the best single opportunity in the entire story to show some diversity. (Well, I guess Radagast could easily be of different ethnicity too, if he shows up.)