Ya, it's not true that it was all written in advance. JJ Abrams has said that they always knew some elements, but that nobody could have predicted where it went. But that is also why they picked their own end date a long time ago now, so they could write the story they wanted, not be dragging it out pointlessly. Wasn't it pretty well stated that nobody was actually coming to the island, but that Jacob wanted Jack to see the lighthouse? I'm not sure what role, if any, Desmond will have yet this year, though I hope he does. At the moment, it seems possible they are setting up for Jack to be the new Jacob, and Sayid to be the new smoke monster, but I'm guessing it's more involved than that, and hope it is as well.