I think the plotline for season 6 will go something like this - at least the beginning. I was watching "Flashes Before Your Eyes" and I was thinking about what happened to Desmond. There are many other theories like this one, I'm sure, but I thought I might post a different, hopefully easier way of explaining it.
Here it goes:
So when Desmond turned the fail safe key and blew up the hatch, the island sky went all purple, and Desmond mind-jumped back in time and got to relive part of his life. He met Mrs. Hawkings, however, and found that he couldn't change anything - whatever happened, happened. He was hit in the head at the pub, then mind-jumped back to the present.
When the '77 losties detonated the bomb in the season 5 finale, this had a very similar (yet very different as well - maybe because of Jacob's touch) effect on them like it did for Desmond. I believe they will mind-jump as well, but to an alternate timeline where we see what would have happened if the plane lands at LAX. The bomb didn't change anything in the original timeline, however. It WAS in fact what CAUSED everything in the first place. After the "incident', the hatch is still built, Desmond still goes to the island and crashes flight 815 and the whole loop happens again up to the point of the season 5 finale. WHH.
The kicker is that the bomb simultaneously sends the losties PHYSICALLY to the past - wayyy in the past. It sends them to a time shortly after the Black Rock has just come to the island.
The losties will first spend a short time in this alternate reality where the plane lands safely. They will slowly start to remember random memories of the reality they spent on the island, and they will begin to see that although they did not end up on the island this time, their lives and events are still leading towards a similar fate. As they are coming to this realization, something happens and they mind-jump back... and wake up to find they are in the time of the Black Rock - where they encounter the crew of the ship, Jacob (of course) and possibly some original Others.
I was thinking the rest of the show could be about what happens to the losties in the Black Rock times (conversing with Jacob, getting some back, back, back history on the Black Rock, Jacob, Smokey and MIB, and of course the island) and Jacob eventually getting them to the time they need to be in - meeting up with the rest of the Ajira flight people and Richard in the present, just in time to stop Flocke and his team and have a final showdown.
At the same time, starting near the beginning and then throughout the season, the story line of what's happening with The Ajira people, Richard, and Flocke will be happening. We will get backstories on Richard, MIB, Ilana, and maybe Bram. This story will build up to a very climactic situation - and this is where the '77/BlackRock losties will time-jump to (via a much newer-looking, donkey wheel), and then everyone will have a rumble in the jungle for the fate of humanity and the island!
Team Jack vs. Team Flocke
the end :)