The big question for me related to the premiere is: what happened when the bomb exploded?
My theory is that we're going to see a split into alternate realities (timelines), which I'll identify as follows:
Timeline A: the current reality, the one we've been following for five years
Timeline B: the timeline where the plane doesn't crash.
More on Timeline A in a minute. I want to talk about Timeline B.
In Timeline B, the plane didn't crash. The people on the flight end up in Los Angeles, and their lives continue as they were going to continue.
It's my theory that we'll see this timeline in some form. I'm not sure if we'll see it right after the landing (2004), in the "present day" (2007), or both. It's also my theory that it will be absolutely miserable for everyone involved. People might even die.
The other interesting aspect is if the plane never crashed, then that's because either Desmond's failure to enter the code never happened, or because the magnetic anomaly was indeed destroyed by the hydrogen bomb. This is very interesting because it could have some long-term repercussions for the Island the Others. Is Desmond still on the Island? Did Widmore find the Island after the bomb blew in 1977? Did Dharma Project still exist? I don't know.
In Timeline A, there are currently two "settings". One in 1977 (the LOSTies in the past), one in 2007 (Fake-Locke, Ben, Jacob, Sun, Richard, etc). This timeline will continue to be shown. But the question is: where? In 1977, or in 2007?
My theory is 2007. This is because I think that 1977 dosen't have anything else to offer the storyline. The characters were in 1977 because they had to join up with the Dharma Project and, in some sense, cause the Incident. I don't see any reason why they should continue in 1977 because that would mean the 1977/2007 split will still have the be managed, along with the Timeline split. Too much to manage.
So I think they'll be catapulted to 2007 and be on the island in the same timeline as the people at the statue. The events that happened at the status, where Jacob was killed and Locke's body revealed, are events we have to see. It's my opinion that we'll see this storyline play out as the primary Timeline A storyline.
Finally, the two timelines have to be connected somehow. Otherwise, why have the split? As to how they're connected, I'm not too sure.
One guess is that some people will have their consciousness moving between the two timelines. This would provide a nice realization of how messed up blowing up the bomb left things and give a problem to be solved (fixing the broken timelines).
Another option is Jacob. Jacob is dead in Timeline A, but alive in Timeline B. Perhaps he can somehow help unite the timelines and fix all the wrongs, at the cost of having people assist him in his conflict with Fake-Locke (aka Man in Black).
I'm not sure which one I'd pick of the above. Maybe it will be both. Probably it will be neither.
And that's my theory as to what happens. In thirty minutes, I'll find out how right (or, more likely, wrong), I am. Maybe.