I think it's pretty clear that next season will not be present time vs. flashback/flashforwards, but instead will be on the island vs. off the island. They will probably even still use the woosh to signal that they are switching gears.
I think the island has actually been moved. Maybe some of the bad things that happen are a result of where it has been moved to, a different climate perhaps, such as a cold one.
The artifacts on the island, hieroglyphs, four-toed statues, ruins, make it seem like the island has changed geographies before, and of course they imply that there was an ancient advanced civilization on the island. I really do think it is going to turn out to be Atlantis.
With Locke being pre-ordained and certain objects from the island having belonged to him seemingly in a previous life, I wonder if they aren't taking a page from Babylon 5 and having Atlantean souls being reborn in off-island bodies, like Minbari souls being reborn in human bodies. That would certainly explain Locke. It would also explain why the O6 have to go back to the island to set things right because they belong there because they, like Charlotte as well, were "born" there. It would also explain all the others lists. The lists of "good" people are of people who belong there because they were "born" there. Anyone who has been allowed to die or leave the island without it being required that they come back (Desmond, Ben, Michael) don't have Atlantean souls. Crazy?