Haha. White people don't like it when their white characters are portrayed by non-whites... it takes away part of what helps them identify with the character. Even as a non-white myself, I'm not THAT pressed, but The Rock can't really pass for white, and Captain Marvel is supposed to be a little white boy that grows into a big white man. That kinda small town bumpkinness is thrown off when you add in non-whiteness.
That said, the Rock would OWN as Black Adam, and I think he'd enjoy the role quite a bit.
Now, with Captain Marvel not being a thoroughly deep character, I think John Cena can actually pull it off. I've seen the Marine, and let me tell you, his acting skill isn't that deep, but Captain Marvel doesn't require much acting skill, it requires a big build, a big smile and a square jaw, and some stunt work... Cena can do that, honestly. I'd enjoy seeing him as the big red cheese...
"Captain Marvel" isn't about Cap... it's about Billy. It's important to get a skilled child actor for Billy (not to be confused with a teenage actor), but Cap is a big cheesy superhero, and honestly, based on the Marine and his wrestling performances, John Cena can do that in his sleep.