In the meantime, carry on arguing for casting actors who are wrong for the parts you're suggesting for no reason other than stunt casting.
How do you know he's wrong for the part before we've even seen the movie?
I once saw James Earl Jones play King Lear on stage. Trust me, he was great.
Don't tell me Heimdall is more iconic is Lear!
I'm not bothered about Heimdall. Personally, I'm just going to complain that all of the Asgard characters are played by humans.
What bothers me is why people want to see Will Smith as Superman. What purpose does it serve other than saying "look at us, we're not racist, we cast a black man" and generally making people think that they've done something good ? Let's all give ourselves a pat on the back because we're soooooo progressive.
Directors don't cast Will Smith in a role because it's some progressive move, directors cast Will Smith because they want to make money. You're not doing anything clever by casting him.
If, on the other hand, you cast some black unknown then why are you doing it ? It's just casting a black guy to stir up controversy and create publicity. You'd be exploiting whatever poor sod you ended up casting. For the rest of his career he'd be known as "the black guy who played Superman".
Like it or not, the visual depiction of a comic book character is as important to that character as how he is written. He is not a character in a play or a book, he's a character from a visual art form.
In this thread I've read one argument against doing the reverse, casting a white man as T'Challa for instance, is that being black is part of their character - and that is the problem. Until you stop making black heroes for black people and start making black heroes that everyone can identify with you will always limit the potential popularity of that character.
All you do with this kind of casting is temporarily manage a symptom, not cure the underlying cause.