All right, so I'm updating my theory after
Recon. Really, it's a merging of various theories already posited. First of all, I'm changing my stance to the so-called alternate universe really being purgatory, as in, the characters are dead. Well, okay, still not the Catholic idea of purgatory, but a land of holding for the dead all the same. I backtracked on that before. Now I'm backtracking my backtrack.
The "sideways" flashes really are in the future, after the massive bloodbath
Temis is gleefully looking forward to. I've always suspected that they would use the sideways universe in a scenario where a major character, like Jack, Kate or Sawyer, would be shockingly killed off, but then their sideways counterpart would step into our universe in a later episode, ala Starbuck or what's-his-face on Terminator: The Sarah Connor Chronicles. Now I'm expanding that idea to all of our characters. They'll all die.
But this will be a couple of episodes before the finale. The twist is that after they die, they'll enter the "spirit world", which the Island is a gate to. This is why Jack has a cut on his neck, an echo of the wound that kills him. This realm is the source of the whispers. This is why ghosts appear on the Island. This is what Jacob is really guarding. This is what notLocke really wants to be free of. He's a man who died, but learned to become powerful enough as a spirit that he can interact with the real world. This explains his connection to the dead, his transformations, etc. The last episode will be the Candidates getting out of the Island's land of the dead, which looks like an alternate world where the characters have made better choices, a trick to keep them there by the Man in Black or Jacob. They escape and come back to the Island, defeating whoever is evil once and for all.
This also leads into the unseen seventh season, the zombie season. After all, since all of our characters have come back from the dead in the end, they're technically zombies.
