Kojima tweets rather openly about the Metal Gear Solid Ground Zeroes and Metal Gear Solid Phantom Pain, so this tweet is definitely not related to them, but you can never be certain about him; he does like to over exaggerate from time to time.
It looks like it might not be a new Zone of the Enders title either, because that project was canceled and the team was dissolved. Perhaps a new MGS rising game, or Boktai or a new IP or maybe it was just sarcasm.
The title he is currently working on is Metal Gear Solid Ground Zeroes which will serve as a prologue to the actual game Metal Gear Solid V: Phantom Pain. Both of these titles have blown our conventional understanding of the MGS series to bits.
These new titles are open world games where player is given a free choice to explore whole a lot of options to complete the mission. You will be able to do it silently or you can go all gun blazing, and you will be able to commandeer vehicles and horses.