A black Elsa? Imagine if they made Mulan, Pocahontas, or Tiana white. There'd be a riot. "But there's a little dragon, a talking tree, and talking animals! Why can't she be white?!"
I'm not against a black Elsa as long as there's no double-standard against white people. If you want to represent other races, do so in their own stories. Disney gets it. Mulan was in China, Tiana wasn't out of place (the US isn't natively white or black), and Pocahontas was indeed living where her people had lived for many, many years.