How will it end?
Maybe we’re watching the actual end in the flash sideways right now. Perhaps that the real cause of the non-island universe is something that has yet to happen, and the story on the island now is how the losties kept the crash from happening and sunk the island. This will allow us to see both a long epilogue of what happens to the characters without the islands interference, and a satisfying explanation of how/why the island exists and why they are involved with it.
So, in a way, we’re actually seeing flashbacks still, but the flashbacks are of the losties on the island, we just don’t know that its something that already happened yet.
Now that, would be a blow-your-mind-out-the-back-of-your-head moment! I hope I’m wrong though, just because I want to be totally surprised.