NeoGAF users have spotted a trio of articles mentioning that the prequel to Metal Gear Solid V: The Phantom Pain - Metal Gear Solid V: Ground Zeroes - is due for release on 18th December 2014. The news comes from the Igromir Expo which is being held in Russia right now.
Ground Zeroes is already available on consoles, and has been seen March this year. But, there's been no official word on the PC release date since it was confirmed it was actually heading to Steam. So, take this news with a pinch of salt until the devs officially announce something.
Ground Zeroes is a prequel, or prologue, to the upcoming Phantom Pain which is due for release some time in 2015.
A few gameplay videos of The Phantom Pain were shown off during TGS 2014, and English versions for two of them were uploaded last week - check the out over here - they show off Quiet, the deadly silent sniper, and Diamond Dog, Snake's furry wolf companion.