Rodrigo Santoro-
IMDB Page
Rodrigo Santoro is probably most well known for providing the Brazilian language voice dub for the titular character in
Stuart Little (1999) and its sequel. Wait, I meant he's most famous for playing the villain Xerxes in
300 (2007) and its sequel. Sorry, I sometimes confuse those two franchises.
While Xerxes and Melkor aren't the same type of villain, Santoro has experience with playing the baddie. He's also an attractive actor with multiple telenovelas and romantic comedies under his belt. At 40 years old, he's not too young as to make the Melkor-Manwe/older brother-younger brother dynamic seem laughably unbelievable. But he's not too old that in a few years he won't be able to throw down with Fingolfin.
The main complaint I see people having with casting Rodrigo Santoro is that he's not pale. As I've said in other casting nomination threads, paleness is not a measure of tone but of intensity. It's a cool idea that Melkor becomes paler as he spirals out of control. And the darker Melkor starts, the paler he can get. A lighter skinned actor becoming quite pale is interesting and narrative. A Brazilian actor becoming quite pale is more interesting, more narrative, and a little creepy- almost supernatural.