So right now, all we have is Hideo Kojima
confirming that after Peace Walker he's been focused on something named the "FOX Engine." Metal Gear Rising was supposed to be on this engine before it was handed over to Platinum Games
. There have also been a few tantalizing screenshots.
So what is this game that's being linked to the FOX Engine?
Metal Gear 1 or 2 remake
There's an Abrams tank, although I'm not sure if that specific variant existed in the late 80's / early 90's, and a guy wearing gear that seems reminiscent of Snake in the first two games.
Yet Another Big Boss Game
It means new story material while still fitting with what the screenshots seem to depict.
Just a tech demo
game that's supposed to show off the FOX Engine is far ahead.
Metal Gear Dating Sim
Because see-through clothing, that's why.