So let's get the basics started; We know from the trailer and the conversations that some type of conflict will be in Skyrim. From NPC conversation's it seems that Orc Mercenaries will be aiding some Skyrim soldiers but to what purpose we can only speculate. (I personally think that Skyrim will be in the state of civil war which will be encompased by a Seperatist faction and a Pro-Imperial faction.)
What we can see from the trailer is that this conflict will also involve dragons, and the destruction that will inveitably follow as they return. But what else can we speculate from the trailer and Oblivion?
More from the trailer is that this "DragonBorn" (Not to be confused, I believe, with the Septim blood line), will be our ever so fated hero to save the world...again. Though other then that, we don't know why s/he's called the DragonBorn nor do we have any idea whether this is because s/he was born under a star sign or maybe born under something else. Maybe a blood line, we don't know. (Yet)
Though this might have been planned along time, from prior to the Oblivion crisis. We also know the Elder Scrolls spoke of it, so it is far ancient history preceding the Alyeids. Although, if so, are we to assume it'll be somewhat like Morrowind's plot? If so, I'll be looking forward to it.
So any speculations from y'all?