MGS4 was not ground breaking, lol. It retconed a lot of the series and gave half ass explanations for things that didn't need them. It also threw in a TON of characters that held no real weight on the main story. Mei Ling, Johnny, etc. These characters were just thrown in there for fan service. I'm by no means saying MGS4 was bad, it just wasn't "ground-breaking".
As for a Gray Fox game, ehh, it's not really necessary. He's not THAT important of a character to have his own game. It would be something similar to Rising if anything. Which a lot of you guys don't like. A game that would be great however, would be a game where you play as The Boss alongside the Cobra Unit. The Boss is the reason behind the series after all.
Also, I do believe Ground Zeroes will be ground breaking. It's the first open world experience in the MGS series. Combine that with borderline next level graphics, multiple ways to tackle a mission, tons of weapon unlocks, motherbase building/customization, and tons of other shit we don't even know about yet. This game will definitely be one of the best in the series.