Sandbox games typically have a lot of things going on simultaneously, and in order to maintain stability in gameplay, they have to sacrifice visuals for a more open-world experience. I just don't see how Kojima can get around that - regardless of how much KojiPro is trying to advertise the new FOX Engine. MGS V might look good in the trailers, but I doubt it'll look that good once we actually start playing it. I expect that when a lot of things start showing up on screen, the game will progressively degrade the graphics in order for it to run smoothly; just like ever other Sandbox game that I've ever played.
But, like I said before, I don't think MGS and Sandbox should have ever been mixed together. I'm all for a linear storyline, with old-school infiltration missions, in highly-secured indoor environments. I'd argue that's one of the main things that has made MGS as successful as it is today.
And I hated the extra ops missions in Peace Walker. The weapons were laughably ineffective.