If the information in this email -- or even the information about the email -- is accurate, it could be a hint about Zone of the Enders 3, which has evidently in Hideo Kojima's thoughts recently. It's got a 3 in it! But if we're speculating, we figure we might as well go for broke. Maybe it's a new Snatcher game! We never heard of anything coming out of the Kojima/Suda 51 "Project S" collaboration except for radio dramas -- the pair could still be working on more Snatcher-related material.
Other possibilities -- which we must include lest a Kojima post go by without mention of Metal Gear -- include the trademarked Metal Gear Solid arcade game, and, if it's a different project, "the next MGS." And let's not ignore the possibility that this isn't true.