According to an article from http://www.computerandvideogames.com David Hayter, the iconic voice of Solid Snake, and Naked Snake of the Metal Gear Solid series, wasn't even APPROACHED for the role of Big Boss in Metal Gear Solid 5. Kojima gave a response saying "It's a new type of Metal Gear game and we want to have this reflected in the voice actor as well,"
Hideo, you can re-imagine Metal Gear all you want, just leave the voice-cast alone. In Twin Snakes David Hayter gave up HALF HIS SALARY to let the entire original cast of MGS reprise their roles, we've had the same voice-cast since the beginning for every game they were in, this is one of the ONLY series that has such a stable cast, to screw that up now is to completely throw away everything that's been worked for since Metal Gear on the MSX2. Maybe this is what Kojima meant when he said Metal Gear Solid 5 would be unsellable, not that it was going to be controversial in content, just that he was going to royally tick-off the entire fanbase to the point of insanity. Metal Gear Solid 2 had Raiden as the main character and MGS3 didn't even feature Solid Snake, but they both introduced new things and kept what was familiar and what made Metal Gear Metal Gear. Too much innovation will hurt MGS5 (Which is apparently going to be two games) and so will changing voice-actors. What I liked personally about the MGS sequels is the gameplay and cast put into new situations while still unfolding the storyline piece by piece, and I'd enjoy a completely new Metal Gear game with open-world and stuff like that as long as it's done right and as long as we get to have the same familiar voice-cast. Nobody ever complains "This cast has gotten too familiar to me, get someone who sounds nothing like anyone who's ever been in the role" (I'm looking at YOU SEGA) and nobody ever should, that's as dumb as saying "We've got a new development team, let's re-boot Devil May Cry instead of re-imagining Mega-Man" (You know I'm talking to you Capcom). I fear that MGS5 won't be very good since "Open World" usually just means "Padded", but this IS Konami we're talking about, creators of Castlevania.