Stupid ass washing machine...
I've given the story direction a lot of thought since pissing my pants (literally, I'm not ashamed to say it) at the reveal today at E3 and I've come to the conclusions as most of you - and it's this un-related thinking of drawing the same conclusions that makes me think it's the most logical step forward in terms of plot.
As we know, and have known for four frustrating years, at the end of Halo 3 Chief was floating starbound towards a mysterious Forerunner planetoid. The trailer unveiled today showed us that we were right about that and seemed to show the Foward Unto Dawn approaching what - to me - looks like a door, most likely a portal within the Forerunner planet.
Now, at the end of Ghosts of Onyx it was revealed that the planet Onyx was actually completely Forerunner made and underneath the crust of the planet was the shield world - a bomb shelter from the Halo Array, accessed via a portal. To sum it up quickly, Fred, Kelly, Linda, Mendez, Halsey and S-III Team Saber are stuck there in the Micro-Dyson sphere with no way out.
My theories as to Halo 4 are that Master Chief has been taken to Onyx, or what remains of it. The portal-like object we saw the FoD drifting towards could be a way in for John.
One thing I'd like to raise at this point is the FoD's state of disrepair - it's exploding, and is on fire. It's breaking up, heavily. At the end of Halo 3, it wasn't like that at all. My theory on this is that the Onyx Sentinels - the same ones that comprise the planet Onyx and attacked and decimated UNSC and Covenant fleets - have been harrassing the FoD and that's why Cortana needs John to wake up.
As such, then, I conclude that in Halo 4, John-117 and Cortana are heading towards Onyx, where they will enter the shield world and most likely encounter the others trapped there, and uncover some hidden, sinister truth about the Forerunners, and John's "destiny" will be confronted.
TL;DR - read it, you uneducated slob.