It appears Phantom Pain might be coming in the next year if a recent Reddit thread is to be believed. User Hitzkolpf has realized that if you register your copy of Metal Gear Solid V: Ground Zeroes it will trigger a message that says you can return to get bonus DLC for the next game early 2015.
This would go along with Kojima saying to not expect the game any time soon as there was a lot of work still to be done on it. When IGN spoke to him recently he expressed concern over how big it was, saying it would be “to big to finish.” He also said that it is over 200 times the size of Ground Zeroes. This suggests that while the main story for GZ might be relatively short, the main story for the full game will be massively long. Lets hope this is true, because GZ will not hold most of us MGS fans for long.