For example, Illyana Rasputin, it's not going to look the part if she's not a blonde Caucasian. The same is true of Danielle Moonstar, needs to be someone who looks the part. Both of them have a clearly defined ethnicity.Aquaman is defined as being from a kingdom in the Pacific Ocean. Jason Momoa is Polynesian. Kinda fits. Yes, I know that he doesn't look like that in the comics, but the other Atlanteans often have been.
Heimdall is similar, he's defined as being Asgardian. Non-white Asgardians are a thing in the comics, so the only stretch is how he can be the biological brother of Sif, who doesn't look like him.In the comics, Johnny and Sue are biological siblings, changing that is what the problem was, not the color of their skin.Chris and Jessica weren't a stretch. Sure, they're not similar enough that I'd peg them as siblings seeing the two of them in a crowd, but I'd seen actual siblings less similar.