I do foresee some difficulties with casting Elwë as a different ethnic background than the other elves.
His brother is Olwë. Whose daughter Earwen marries Finarfin and becomes the mother of Finrod, Galadriel, etc. All BLOND kids. So, if we cast Elwë as someone black, we should follow through and also cast his close family members (including Celeborn, Galadriel and Arwen) accordingly.
There is no reason we can't do that. I really do have to say that - there is absolutely *no* reason we couldn't cast a black woman as Galadriel. It would be...unexpected, sure, but who is to say that elves can't have blond hair and dark skin? Elven hair color is well documented. Skin color..rather less so.
Corey Olsen's main complaint with ethnic diversity among the elves for casting was that....they all start out in one place. They are at Cuiviénen together. And *then* they split off into these different groups for various reasons. So, if the split happened along racial lines....why? Why would the Asian elves suddenly break off and make their own group? It....wouldn't make sense. It would make the divisions of the elves look not quite like splintering but more like self-segregation. And why do they even have distinct racial groups if they're all living together in one place?
So we're left with either more-or-less homogenous elves (with diversity for the Men and possibly Dwarves), or random minority families scattered throughout the elven kindreds. The Noldor, who are mostly dark-haired, could have random black or Asian families mixed into the group and it wouldn't necessarily stand out as weird. It might be harder to do that in Doriath, but should be doable. Part of me really likes that approach - having some low key diversity that the elves don't even notice. BUT...there are some implications to deal with. And one is that if all of the non-white characters are unimportant background elves without a role or speaking part....yeahhhh, that's not exactly a good thing. And if we do make use of a more diverse cast in the main roles, then we have to trace the family tree so that people look like their parents' kids. Elwë and Finwë would be the worst candidates for that, because they're related to everyone. MUCH easier with a character like Círdan, Mablung or Beleg, where there are no family members to worry about. [It would be somewhat amusing to me if Círdan were black, and every time the elves talked about him, they mentioned his beard - as that is obviously his most distinctive feature!]
Also...I really do want Elwë to be over 6' tall. We can't always make him stand on a box.