Kojima suggest new “devil project”

Kojima has left a cryptic message on his twitter account referencing a new “devil project.”

The tweets read:

“Have to give priority to another line’s scenario etc., so now I cannot work on ‘the devil project’ and ‘writing’.”

“Worked on my ‘devil project’ a bit after lunch. An experiment went well. Plot adjustment. Additional project. Phew.. now another line.”

Could this be a Zone of the Enders 3 or a new Metal Gear game?

Thanks to VG247 for the info.