I mean his physicality is really the only thing he has to bring to the role (at least over the other picks, its the only thing that makes him stand out) not to say he can't bring any type acting skill too it, as well...
{but, if they want a physically imposing Doom} I could see that
he also has a strong look for "human" Doom (in any type of flashbacks/ origin story) type scene
{if they really want to play off his vanity of a guy who was very obsessed with his looks... so, to have that taken away from him drives him mad} kinda way
so, saying he'd only be a voice over, seems counter to that
if the character ends up being CG/ or just stuntmen in the suit and mask for most the time... anyone can voice that (without having the looks or size for it)