SPOILER ALERT: Please note that some major spoilers will be revealed about Stephen King's "The Dark Tower" series--if you haven't read the novels yet, I would stop reading now!
In one of the early seasons of Lost the writers admitted that Stephen King has been a huge influence for them. This began a firestorm of, "Is Stephen King behind Lost?" and "What is Stephen King's relationship to Lost?" arguments. Stephen King himself praised Lost on his website, which furthered the debate. Ultimately, regarding the way the show will end, I believe that Stephen King has already paved the way.
For those of you who are familiar with Stephen King's work, you might have noticed a number of striking similarities between characters in both The Dark Tower and The Stand. You have Charlie Pace/Larry Underwood & Eddie from The Dark Tower, Jack Shephard/Nick Andros & Stu Redman, and possibly even Roland Deschain, Kate Austen & Rose/Susannah, Walt/Jake, and even Vincent/Oy. Even the seeming antagonist of the story of Lost, the Man In Black, shares the name with the true identity of the antagonist of The Stand and the exact name of the man Roland chases throughout most of The Dark Tower series.
With all of that established, one need look only at the eponymous seventh book in the Dark Tower series for a clue as to how I believe Lost will ultimately end. In the final "episode" of The Dark Tower (if you will), the favorite and beloved characters are killed off one-by-one, including Father Callahan, Eddie, Jake, Susannah, and even Oy--all as a result of Roland's unerring quest to reach the Dark Tower. We can speculate that, as Roland and his ka-tet battled the forces of evil (of which the Man in Black claimed membership), so too will Jack, Kate, and the rest of the gang confront the Man in Black on the Island. The odds are that, much like in The Dark Tower, most of the favorite characters will be killed off, including Kate, Sawyer, and Sayid. My guess is that Hurley and Vincent will be saved but who knows!
The alt-reality/flash-sideways that we are witnessing seems remarkably similar to what Roland sees after he enters the tower. Essentially, all those characters that died during the Dark Tower are living in some parallel world, happily ever after. Doesn't it seem plausible that the same could be said for Kate & Sawyer? (Or maybe Sawyer & Juliet?) Rose & Bernard? Sayid/Boone & Shannon? Daniel & Charlotte? Etc.!
Ultimately, we will be left with Jack and the Island, much like we were with Roland and the Dark Tower. Perhaps the Man in Black in Lost is more of a parallel for the Crimson King, in which case Jack will somehow face the MiB on his own and defeat him. The fact that Christian Shepherd has appeared so many times (and we are lead to believe that the MIB has taken his shape more than once), I wouldn't be surprised if the ultimate confrontation on the island was between Jack and his father (either as a phantom created by the MIB or the MIB himself who truly turns out to be Jack's father).
As Roland scales the heights of the Dark Tower and realizes only at the last minute that he had performed this same action countless times before, so will Jack perform some sort of act that he will have committed an infinite number of times. My guess is that he will destroy the donkey wheel that moves the island and in so doing will mess with the balance of space and time and find himself either back on Oceanic 815 or on the island itself.
What then would be the purpose? In The Dark Tower, Roland sacrifices people left and right in his endless pursuit of both the tower and the Man in Black. Upon reaching the door with his name on it at the top of the tower, he realizes that he had missed out on love and friendship and he regains some of his humanity. Doesn't this seem similar to Jack's sudden change of heart over the past few episodes? He went from being narrow-minded, operating with tunnel vision that was fixed in on leaving the island, then getting back to the island, and then on nothing in particular. Now, he seems to have softened a bit and has opened up to the possibility that he DOES have a destiny, that it is bigger than him, and that it has given him some sort of purpose. Maybe, in the end, the story will be about Jack (much like the Dark Tower was about Roland) recovering some part of himself and, when he resets for the final time, we will know that, THIS time, he will succeed in saving himself (much like we can assume that Roland will during his final sojourn through the desert).