Should a minority actor be allowed to play a character that's traditionally depicted as caucasian? The answer to this question is simple. Just ask yourself, Does the characters history SPECIFICALLY implicate the characters national origin.
Thor.....Norse god of thunder. Probably should remain caucasian.
The black panther...who's origins are from Africa; should probably remain black.
However, to imply, like some of you have, that Halle Berry shouldn't have been allowed to play catwoman is nonsensical. For what reason? Catwoman, is simply a woman. Her origin story is universal and could be applied to anyone of any race. Hell, there could very well be a latina catwoman, or an asian american catwoman. The fact that the movie was horrible had less to do with Halle Berry's skin color and more do to with the horrible creative choices made by those in charge.
And this whole example of "what if we cast a white actress as storm" mess is nonsense. If you've ever picked up a comic book you'd know her character origins are rooted in Africa. It obviously wouldn't work.
Somebody was actually STILL griping about Michael Clarke Duncan being cast as Kingpin. WHY? There's nothing in that characters origin that prohibits him from being any race. And frankly, I felt MCD was one of the only redeeming things about that movie. SO RELAX already.
As stated before, a lot of these characters were created in a time when minorities were still getting lynched from trees in the deep south. We've come a long way since then and I dont see why a black actor couldn't depict any superhero who's race isn't specifically implicated.
Now, do I think Beyonce is right for Wonder Woman? No. It'd be hard to pass her off as greek or Amazonian. Wonder Womans ethnicity is actually more apart of her character than some of the others. HOWEVER, I would not have a problem with miss Beyonce if she wanted to play Supergirl or any of the other DC comics Heroines.