What we already knew:
1. From the Halo 3 Legendary "teaser" we know that there is an object that appears to be an artificial planet of Forerunner construction.
2. From Onyx, we know that Dr. Halsey and crew ended up in what they think is a dyson sphere, location unknown. (PS they are not ON Onyx, Onyx was destroyed)
3. From Cryptum, we know that the humans and prophets had an alliance and lost a war with the Forerunner. But the Forerunner don't "hate" humans, I think the idea is that the vanity of the Mantle clouded the judgement of many Forerunner which lead to their downfall at the hands of the Flood and their own technology. I believe this is the important story thread that we are getting from the Cryptum series.
4. Karen Traviss of Gears of War fame has been contracted by 343 to write a new series of books based on the Onyx storyline. If you are a Halo story fan and you haven't checked out Traviss' Gears books, then I highly recommend that you do.
What does the Halo 4 trailer tell us:
1. Something suddenly changed. Cortana's panic tells us that she did not anticipate what was happening. Merrily floating along in space roughly toward the unidentified spherical object, the sphere suddenly comes to life and begins to pull the ship toward it, breaking it apart. Cortana acts quickly and initiates some sort of "fast" wake-up for MC because they need to leave the ship "right now".
2. Armor changes, weapons changes, etc are pointless to discuss vis a vie the storyline as those things change from game to game. I mean, how the hell was the armor on Reach so advanced with jet packs, shields, cloaking, etc. since it was set in the past? Why didn't MC have those options? Well, just because, lol... it's irrelevant to storyline really.
Rampant speculation:
1. I can only hope that Karen Traviss is writing the storyline for the new Halo series. If so, then we can probably figure that the unidentified sphere and the dyson sphere where Dr. Halsey went are indeed one and the same.
2. This storyline has the gameplay advantage of developers getting to mix Spartan IIs (MC and Kirk) and Spartan IIIs into the same game.
3. Since there is evidently a new trilogy of games planned, I'd be surprised if the Elites weren't somehow worked back into the equation; as well the Prophets and whatever is left of the Covenant.
4a. Perhaps the humans and forerunners will be allies.... the first Cryptum book begins to introduce the Precursors into the equation, they may be the enemy.
4b. Perhaps the Forerunner who fled to the dyson sphere are ones who were responsible for "devolving" humans and prophets, and still haven't learned the lessons of their vanity... they may be the enemy.
I like my first idea best, a Precursor enemy responsible for bring the Flood to the galaxy who desire to take back their place as masters of the galaxy. I'm certainly not ready to say that 343 is inferior to Bungie, so far I've enjoyed the first Cryptum book, I think it's an amazing move to hire Karen Traviss, AND I've been eagerly awaiting a new Halo series featuring Master Chief picking up where Halo 3 left off...
EDIT: Just read this Halo 4 description on Waypoint...
"MASTER CHIEF RETURNS TO CONFRONT HIS OWN DESTINY AND FACE AN ANCIENT EVIL THAT THREATENS THE FATE OF THE ENTIRE UNIVERSE"
Hmm... just might be right about that Precursor thing, lolz...
[Edited on 06.07.2011 4:12 PM PDT]