Very interesting. Who has the most to gain by revealing their title this close to E3?
My guess is it's a third party that was jockeying for reveal at either Sony/Microsoft - but got pushed aside for time. So not anything massive, but still 'AAA'. Following that logic, I could see it as the rumored new game from Volition - or Injustice II (which has pretty much been confirmed at this point via Ed Boon's Twitter icon).
Alternative is something big that would get a gameplay reveal at a stage show. Would this be the way for Resident Evil 7 to stand out? Maybe it's how Ubisoft makes people care about Watch_Dogs 2?
I like announcements like this when they're just a few hours away. Very manageable.
Edit: Oh, so it's Agents of Mayhem. Cool, been wondering what that thing is.