Maybe Ground Zeroes and The Phantom Pain are in fact the same game, which is MGS5!
They look exactly the same graphically and Kojima did say that Ground Zeroes was a prologue to something, by prologue he might have meant the 'beginning point' to all these teasers he's been throwing around, more are most likely to come before he pulls the plug.
When you think about it, would he really reveal two different MGS games that are being developed simultaneously?
Well anyway I'm calling it, they're the same game
Every Metal Gear game has had a subtitle to it even MGS1 with the remake (Twin Snakes), you got Metal Gear Solid 2: Sons of Liberty. Metal Gear Solid 3: Snake Eater. Metal Gear Solid 4: Guns of the Patriots. Metal Gear Solid: Peace Walker. Then upcoming Metal Gear Solid: Ground Zeroes. Metal Gear Solid 5 could be Phantom Pain.
But saying that it would be silly to reveal MGS5 now along with Ground Zeroes so I kinda agree with Trains that this could be the same game.
If it is a MGS trailer Kojima sure has succeeded in getting every ones attention and after watching the trailer once again I agree the outside looks so much the same so I might be now 70% convinced but still 30% unsure.