Given how unlikely it looks to happen any time soon, I'd say the end of MS's press conference at the start of E3 would be the best place for such a hypothetical unvieling. They have a record of ending their E3 conferences with the unlikeliest of announcements over the last few years.
I mean, two years ago, the chances of DNF being released was compared to the chance of Final Fantasy or Metal Gear Solid coming out on the 360. That annalogy lasted long. They topped 2008's FF13 announcement with 2009's MGS:Rising announcement. How can they top that this year? It's not like there's a Gears or Halo waiting to be announced. Hypothetically speaking, that would be the most impactful time to announce DNF's release
