Recently, I find myself making many comments on other posts along the same line. I'm not sure I've made my point, though, and fear my comments appear as overly negative when I mean them in the opposite way. I offer the following for clarity's sake.
First, I'd direct you to FlashMedallion's post Why children are so important to the island for a great explanation of causal loops and how they might be a big part of all the apparent "magic" on the Island.
I've wondered for a while now whether the writers will wrap up the mysteries scientifically or supernaturally, or if they'll go the middle route. Of course, none of us can be sure; so let's explore them all!
If they solve things scientifically, they might say the Island is a time machine from the future which became stuck in the past. Don't know about you, but I find this - if not a great example of jumping the shark - less than satisfying.
I'm not sure the supernatural explanation would improve on the scientific one, though. I mean, according to Ben, even God can't see the Island - though I'm not sure what that means. Jacob and/or MiB could be divine in some way, and perhaps created all Space/Time from this Island. Could be the reason it exists on Earth. I like this idea more than the strictly scientific one. But not much.
Which leaves us to speculate on the middle road. I find myself thinking the writers will choose this path due to the way Lindelof and Cuse have talked about the ending. Now I apologize, as I'm about to reference Star Wars, which may be too far across the nerd-line for some of you! But the producers have discussed their dissatisfaction with the explanation George Lucas gave in Episode I for the Force. He attributed it to a species of microscopic lifeform living in all cells. In the original Star Wars, however, Lucas opted for a middle way answer: "It's an energy field created by all living things. It surrounds us and penetrates us. It binds the galaxy together." Lindelof and Cuse have said they prefer this kind of answer, which allows for some mystery to persist past the ending of the story.
Obviously, when comparing the length of the above paragraph with the preceding two, it's clear which I'd rather they follow. For that reason I think the Losties are not part of any prophecy; Jacob and MiB are not gods but somehow simply human; and everything the Losties have gone through so far has just been manipulation.
How is this the middle, you might ask? Well, first I'll say: of course these are just thoughts. How could one of us really know the way Lost will end? It just seems they want to explain enough, but not too much. They don't want to be anticlimactic for over-explaining, nor for leaving too many holes. So I think everything will have a simple explanation - that is, they will fit into a broader picture, and we'll just get the picture explained. (This is much like FlashMedallion wrote in the post I recommended above.) Makes sense for them to explain things by not saying anything about the why or how of the Island. We may find out what the Island is, but not how it got here, nor why. Maybe it's always been here, in which case humans would have no way of knowing the answers to how or why anyway. And, besides, the Losties are the most important part of the show. Why else would a time travel story have started with them?
That's the whole point of this post. I think the Losties are special. I mean, maybe some are more special than others, but I think they're all important to the Island. I just don't think they are special for the reasons we've come to believe - or, at least, not only for those reasons.
Take Locke, for example. He's got some sort of "communion" with the Island. Or does he? Could there be other possible explanations for Locke's seeming connection to the Island than that he actually has one? Remember, I'm not saying he's not special. What if he did create his own candidacy for Leader of the Others when he spoke to Richard in '54? Richard told Jack he'd gone to see Locke and didn't find him special. Would he have given up on Locke if Jack hadn't told him not to? Widmore's only answer to Locke's being special was "Because you are" - and that could be a dodge, if it's not the truth.
All the escapades through time the Losties have taken made them important in Island history, true. Plus, I'm willing to bet they appear in Daniel's journal. Many people, from Others such as Ben and Richard, to Widmore and Eloise, to the Man in Black himself, have told multiple Losties they were important or special. But every step they take - opening the Swan hatch, pushing the button, turning the failsafe, leaving the Island, going back, detonating Jughead - seems to make things worse. All these events which different people have been manipulating don't seem to be in the Losties' interest, but somehow they keep allowing themselves to be convinced to go along.
Ultimately, it seems all the Losties - perhaps even Locke - will end up in the present. With Jacob dead and MiB in his own present, it's possible no one knows anything about the immediate future. Which means the Losties (and everyone else on the Island, for that matter) might finally be free to make a choice no one is prepared for. And in doing so I think Locke will save them all. I can't say how he'll come back (I've written about some of my ideas before in other posts), nor how any of the Past-Losties will return to the present (though I've written about that, too). But I think they are important for reasons perhaps no one knows. Which means the Losties are like The Mule in Isaac Asimov's Foundation stories: they're the unpredictable wrench in the "master plan". They will redeem themselves and everyone else - maybe even against both Jacob and MiB - which is why they're important to the Island.
In essence, I think The Losties are not special because of who they are but because of what they're going to do.