Update: Geoff Keighley says he spoke with spoke with Ken-ichiro Imaizumi, a senior producer at Kojima Productions, and was told this is not a tease of a sequel for for 2013's Metal Gear Rising: Revengeance. "Not true," says Keighley.
The original post follows.
The tease came in this sizzle reel (below, at 2:46:39) put on by Sony at Taipei Games Show 2015. Alongside mentions of The Order: 1886, Bloodborne, Uncharted 4 and other known quantities, this brief appearance of the number 2, in the Metal Gear Rising typeface, kicked up a lot of buzz.
Metal Gear Rising: Revengeance was a hack-and-slash variant on the Metal Gear franchise, starring Raiden, who debuted in 2001 in Metal Gear Solid 2: Sons of Liberty, was featured in 2008's Metal Gear Solid: Guns of the Patriots, and made other cameos.
Polygon scored Metal Gear Rising: Revengeance a 9.0, saying it "combines reckless abandon with the beauty of precision." The next announced game in the Metal Gear series is Metal Gear Solid 5: The Phantom Pain, coming this year on PlayStation 3, PlayStation 4, Windows PC, Xbox 360 and Xbox One. For more on that, see Dave Tach's preview from E3 2014.