Jack is the new Jacob, and he has a whole generation of candidates to protect,just like Jacob by Joshua S. Smith

One of the most important scenes I have watched in recent times that reveals some uber-important answers was never shown on the regular show, it was a scene in the LOST:missing pieces segment of LOST mobisodes on ABC.com. The scene involved Vincent, Walt's dog running through the jungle in what is supposed to be shortly before the actual series of events begin in the pilot episode. Vincent approaches a man wearing white sneakers...It is Christian Shepherd! Shepherd tells Vincent that he needs to go find his son in the bamboo jungle and wake him up. When Vincent runs away, Christian says...He has lots of work to do.

First important thing about this scene...Christian is more than likely Christian, not MIB pretending to be Christian like he told Jack, why would he refer to Jack as his son to a dog? The second important thing...From the very first episode, Jack was already predetermined to have a purpose.
Jack's purpose as many have theorized, is not rocket science. It has appeared very likely he will become the next Jacob, as confirmed by his actions and by Sayid saying "you will be the one".
But, what Jack, does with his job is a bit of a science. As I have theorized several times before in older theories, I believe Jack and Desmond will work together as some sort of team. Using time travel to give important people "a little push" just like Jacob did. One such big giveaway to me was the very odd occurrence in which Jack thought that the surgery failed when he did the surgery on Sarah, but then he returns from running in the stadium to find she is miraculously healed. It just so happens before this miracle that drives his life into a tailspin, that he meets Desmond at the stadium(and Desmond just so happens to intuitively guess, that Jack had a woman on his mind). Hmmmmm.
Some theorize that Jack who becomes the candidate, had time traveled to the time of the surgery that had previously failed in another time iteration, and did it himself.
Other things to consider, the possibility that not only did future Jack save Sarah, but he may very well be the secret man she was having an affair with before their divorce. It is possible in some previous iteration, Jack lost her and didn't get her back(more than likely the common iteration we see most scenes of), but then something changed. What changed is Jack began to influence things and came back for the woman he loved after he realized he pushed her away with his own issues. One big giveaway possibly that Jack loved her is the scene in the season finale of season five in which he fights with Sawyer over letting off the bomb or not. Sawyer asks Jack what he gets out of the bomb, and he tells Sawyer, I had her, I had her, and I lost her. Jack never confirms he is talking about Kate, and Sawyer kind of answers the question whether he is talking about Kate or not by saying..."If we do this, you won't know eachother and she'll be in handcuffs somewhere." Jack says nothing and still wants to blow up the bomb meaning he is more than likely talking about Sarah.
Another interesting thought...after Jack saves a person on a bridge and gets hit by a car, he goes to the hospital, and guess who visits him...Sarah...except Sarah is visibly pregnant. And she is veryyyyy pregnant, meaning she WAS pregnant when she lied to him and said she wasn't before. She was testing him by his reaction to that lie, and his relief made her make a choice between him and future Jack. So the David Shepherd we see is more than likely the son of Jack and Sarah (although whether it is Future Jack or Jack is to be determined). And one big line from this scene is when Jack can see Sarah's secret lover across the street, and she tells him "It doesn't matter who he is, it matters who you are not." Meaning that without her leaving him, Jack will never be with her in the future as a man who knows what to appreciate and what he lost with her.
And now we move to the next big thing...Jack will be Jacob. But now, what about the candidates that may be possibly left? For instance, there is no way there would have not been an answer of which Kwon #42 was if there was an answer necessary before they died in the sub. Meaning that #42 is their daughter, Ji Yeon Kwon!!! And here's some other thoughts for everyone to consider...Sawyer has a daughter (# 15)! Claire has Aaron (#331)! And what if Jack's job is kept in the family with David (#23)! Don't be surprised if this show ends with a little bit of the bitter sweetness that D and C talk about. An end with the island being above water again, where the children of the LOSTIES could experience all the powers of the mysterious island, as Jack and his constant Desmond travel through time and protect the remaining child candidates, just as Jacob did!!!!!!!!