First off, check out this theory, and full credit to Joe. I quite enjoyed it whether it turns out to be even partially correct or not, but I personally bought right in.
http://theoriesonlost.blogspot.com/2010/01/locke-theory-by-joe.html
The short of his theory is that MIB (Jacob's Nemesis) is Locke, but not by taking over his body once Ben killed him. In essence MIB was reincarnated into Locke from the moment John was born, and Locke spent most of his life sensing a greater puspose but was unable to put his finger on it. It was only when he crashed on the island that he realized he belonged there, and began working, perhaps subconciouslyat first, fulfilling MIB's goals, eventually realizing who he actually was.
I would like to add to that theory that not only do I buy it as a strong possibility, but that I think the same thing is occuring with most if not all of the other Losties.
They are all former members of the society that protected the island in ancient times, and that is why they are "destined" to be there.
Locke is the only one we can speculate as to his true identity. He is the reincarnation of MIB, who died at some point after the landing of the Black Rock. Jacob won their battle, but regardless of whether we hold Jacob as the good or bad buy, the balance was upset by his victory. Eventually MIB was reincarnated in the body of Locke, who subconciously sought out his "destiny" over the course of his entire life.
The proof that others are "destined" to seek out the island arrises in Charlotte. We know that John suddenly felt at home once he got there, but Charlotte outright admitted that she had actively tried to make it back. There are some characters that you can say wound up in the past because they had to achieve certain acts in order to continue the cycle of WHH, but that doesn't entail a character feeling a need to find the island. Charlotte's "need" to get there, implies that she has some subconcious voice directing her back.
Jack, likewise "needed" to go back during season 4. But Jack had been raised to doubt, and so he fought the reincarnated spirit within him, which was telling him to stay on the island. He fought it so hard that he eventually did leave, but once his subconcious mind had tasted its true home, he wouldn't let Jack live in peace away from the island and Jack came to believe in his "destiny". He still hasn't discovered who he truly is in the way that Locke has, but that is the climax of the show.
If the characters are all reincarnations of an ancient island society, it explains a multitude of questions
First of all, it gives perhaps the most solid reason for why they are important in the first place. If they are simply random people, their signficance breaks down a bit.
Second, it explains why they seem drawn to the island at such an addictive level.
Third, it explain why they all are drawn to eachother, and why they have entered eachothers lives in the past without realizing it. If you understand that they know eachother from a previous life, then it's easy to believe that they've sought eachother out on a subconcious level.
Fourth, it explains the mysterious need to deny information to eachother. You know, the weirdness on the show that is not simply telling everybody everything they know. If they are reincarnations, perhaps Jack doesn't open up to Kate because subconciously he knows that she's on the other team. Or it could be that he's simply confused, like when you have a piece of knowledge that you know you know, but you can't quite remember. Or it could be the reverse of that, in that he realizes he knows something, but doesn't know why, and so he keeps from expressing it.
Fifth, it explains outright why characters like Walt or Locke seem to know things they shouldn't.
And there are other examples.
But you notice too that there are some characters that had to be forced to the island against their will. Jack was prodded, but deep down, his reincarnated self wanted to go back. He just had to get over how his new body had been raised to doubt.
Daniel however, never wanted to go, and I bring this up to hightlight that some characters are not reincarnations. Daniel had to be forced into going and he fought it every second of his life, and while WHH would indicate that he would have made it to the island anyway, WHH really implies that Ms. Hawking would have and did manage to force him into going. It's in Daniel's nature to want to go, because he doesn't have a reincarnated spirit telling him to go.
So why is all this happening now?
Because the balance was undone. Jacob won, MIB was killed. But the protectors of the island don't want the balance undone, and they reincarnated themselves into new beings that could come back and restore the balance.
When the balance was lost and free will allowed to roam unchecked, the statue was destroyed, the military arrived to conduct experiments, the Dharma Initiative arrived, and all of this eroded and threatened the island.
It's not coincidental that most of the Losties were born around this time, with the exception of Locke who was born just after the arrival of the military. Everyone else was born around the time of Dharma's arrival.
And they born then with the understanding the Island's corruption had gone too far. The spirits of the Island's former protectors placed themselves in new bodies, guiding those bodies throughout their lives back towards the island, pushing them through all of the events we've seen, and now in the final season, Sawyer, Jack, Kate, Hurley, and several others will finally open their eyes to who they really are, and their destiny is to protect the island.
So the real question is, is since Locke has taken his place as MIB, who has been reincarnated as Jacob? Jacob just died, and so it would have to be somebody that has yet to be born, or perhaps who was just recently born. Because even though MIB got revenge, the losties ultimate goal is to maintain balance, and so we're going to need Jacob back by the end of it.