Metal Gear Solid: Ground Zeroes at an "introspective" PAX session about the series today. The game's demo was shown recently to attendees at a Metal Gear 25th anniversary event in Japan, and involved Snake sneaking his way through an open world.
So what else is new about Ground Zeroes? It's a "prologue" to Metal Gear Solid 5, says Hideo Kojima. The open world of Ground Zeroes is literally the entire world – a helicopter can be called to help during missions or ferry Snake in and out of mission levels. Moreover, you can customize the music that the helicopter plays when it arrives to help you ("Flight of the Valkyries" played during the demo we were shown today – Kojima says the louder the song is, the more of a chance of you getting caught, so maybe Slayer wouldn't be the best choice).
Kojima reiterates that Ground Zeroes is a current-gen game, though Fox Engine is scalable for next-gen as well. He wouldn't give specifics on the name, but calls the game "a beginning" rather than an end for the series.
Is Ground Zeroes the "Project Ogre" we've been wondering about? That's still not clear. What we do know is it's headed to current-gen consoles (360 and PS3 were mentioned) rather than the next, yet-to-be-determined hardware generation.