See, when I think of Kilmer at his best I think of the way he would just completely inhabit a character more than any particular physical or vocal attribute. He was Doc Holiday; he was Jim Morrison. , he could even pull off a Native American character like in Thunderheart (underrated movie, btw.)
Now, Kevin Spacey is a solid choice as a "voice" man. It's almost like the "voice" guys always play themselves, whereas Kilmer could disappear into a role. Like, there are "Kevin Spacey roles" but there are no "Val Kilmer roles" because you never got the feeling Val Kilmer was playing himself. You know what I mean?