So I love discussing this with my friends and with new info from Comi-con I thought I would bring my theory here for discussion.

So the main speculation from the Lost Panel from Comi-con was that all the videos and previews had to do with alternate timelines.  I.E. Ralph Emmerson's audition for Hurley

http://io9.com/5323521/watch-the-videos-that-explain-the-direction-of-lost-season-six

My theory so far is that the Flash backs of this coming season will be what happened if the plane did not crash.  As the Swan was blown up before Desmond could miss pressing the button, this would mean that the plane never crashed.  So heres where the paradox comes in.  Now it is not confirmed that the survivors or Survivors Prime as I will call them are still existing on the Island.  Back to the Future logic would state that as they were never brought to the island, then they would never have excisted in that timeline, meaning they would fade.

The issue with this is that that would also mean that the Swan hatch was never blown up.  This is too much of a paradox so Back to the Future is out.

Now DC comics Crisis logic would state that this means they now exist in an alternate universe.  Sure that might dictate why we see dead people, (Universe phases, voices heard at one time but not on another) but that would also mean that the multiverse (The idea of multiple universes.  In this scenario 2) Are trying to merge with each other.  This does not seem too plausible as this would take more then one season to spell out and deal with.  Believe me the entire DC crisis series is hard to wrap your head around, and it was only implemented to fix incongruities in the DC universe.

The third idea and most plausible to me is the Star Trek theory.  That even though they went back in time and altered the universe, they are still allowed to exist.  As JJ Abrahms was responsible for both Lost and Star Trek, I can see this logic as being allowed.  This means that Jack, Sawyer, and Kate all live on the island, and potentially ported back to current time on the island but I dunno if that would be the case.  As it is a paradox, it would explain as to how the island can move itself in time and space.  Simply because it is one huge paradox.

So this brings me to a conclusion that no one leaves the island that were already .  This also makes sense since it was very much implied that those that go back to the island wouldn't return.  But as their was an alternate timeline where they never crashed well, everyone lives.  But that means that Jack and Kate never hookup.  It will be interesting to see where the Survivor- Beta group goes as we already spent so much time leading up to learning about their extensive past.  But this also brings some fun points up.

Desmond/Penny- The best episode was "The Constant" where we see desmond and penny finally contact each other.  It is very hard to describe the emotion filled in this epi but if you havent seen this then you shouldn't be reading this yet anyways.  Well the huge question would be, if there was no Swan, then do they still live together.  It was fortold in this first jump back that he was never meant to be with Penny by Faraday's mother.  This could still play back as true as he never impressed her father by sailing around the world.

John Locke- Well Terry O'Quinn already stated that John is very much dead.  But was he just speaking about Locke-Prime.  This means Locke-Beta can still be a grumpy wheelchair ridden crotchity old man.

Radzinsky- Well his head now can't be splattered on the ceiling of the Swan station.  Probably won't hear from him though.

The dude in the hatch with Desmond- Still no word on who he was or what he was doing there.

Aaron- Well My thought has always been that Aaron=Jacob.  Dunno why but it always felt right.  With Aaron Prime being born on the island he could also potentially slip through the paradoxical crack and still exist.  It could also be that Ghost Claire did not want him to return so that he didn't have to live on the island as a Prime, and might faze out.  I have a feeling that the island is the only place not affected by the paradox.  This makes sense as the island never made sense in the first place.  But with the fact that it has time travel capabilities, and if he were to survive as a Beta, well he could spend quite a bit of time searching for the island and then as Faraday says, we have no choice with time.  It could mean that both timelines have to happen in order to exist.  That would also explain why Jacob and random dude speak about how things always repeat themselves.  Aaron would know what happened, and what had to happen in order for the world to be correct.  Time paradox never happen, The world was never corrected, the universe collapses on itself or something. 

I won't be surprised if Aaron-Beta is named Jacob.  This could mean he finds out the truth of Aaron-Prime or survivors Prime and puts the plan in motion.  Either way there is still TONS not explained and I don't like trying to figure it all out as it leaves room for surprise but post if you have any theories.  Remember its more for fun.

The Spock Paradox though makes the most sense.  The producers say that the ending of the series may not be appreciated by all but it is very bittersweet.  I have a feeling that alot of the relationships that were made throughout the series will be shown to never happen, and it will crush many.  I am gonna be mad if Jin/Sun Beta divorce. :(