According to Kotaku, Metal Gear creator, Hideo Kojima stated that Ground Zeroes is an introduction to Metal Gear Solid 5.
The original Snake, also known as Big Boss, is back: and he's the star this time, like he was in Metal Gear Solid 3 and several other games in the series. He can use a helicopter to fly around the world, going between countries and taking on missions.
We're also introduced to a suited figure that appears to be one of the game's villains. The character wears a black, flat-brimmed hat, his face burnt and disfigured. He appears to be part of a group called XOF—FOX backwards—and he calls his squad of soldiers "an army without a nation."