WE, THE GIFTED: 'A "Magic Box" of Gifts'
The premise of this theory borrows from the idea that there are people in this world who truly are gifted.
The island is the backdrop to the 'possible'..it's a place where dreams can come true and where enough will and imagination can manifest all kinds of 'miracles'. The problem with mircales is that in the outside world we often overlook the small miracules or those which seemingly cannot be explained.
I would like to use 3 characters for the initial basis of this theory:
Juliet Burke. A fertility scientist - nothing special about her... except there was - she "created life were there wasn't any". A miracle, one might argue. Infact, in recruiting for Mittelos, Rochard Alpert himself told her that she had a "gift". Very specific words. He alluded to the idea that her "gift" would be multiplied..enhanced..heightened on the island.
Juliet headhunted by The Others..

Alpert knows exactly what gifts the island can heighten..

John Locke. An ordinary man - "meat and potatoes" - nothing special about him, except there was. He survived an 100ft fall and lived..a miracle. Despite being paralysed, he still survived. No mean feat. One could argue that for him to survive such a devasting fall must indicate that he was gifted..that he was destined for greater things. He said it himself - "Don't tell me what I can't do!". And so it turned out, whilst on the island he regained his ability to walk. Consider this - his gift was enhanced..not granted..but enhanced.
Certain death..

But this is no ordinary man. Although he believes crashing on the island was his rebirth, infact, THIS was his rebirth. His gift now apparent by the fact he survived.

Jack Shephard. An ordinary Spinal surgeon - "committment makes him tick" - nothing special about him, except there was. He fixed a womans spine..a woman who scientifically had no right to ever consider walking again. Except he fixed her..he couldn't believe it himself, yet he did it..a miracle. His gift..his natural intangible ability to heal..to fix.
Sarah will never walk again..her spine is crushed..so is her hope.

Jack heals her..it cannot be explained..

That is not important because the man of science, has now become the man of faith. Would his gift translate to the island..would it be enhanced?..

On the island that ability becomes both his gift and his curse - miraculously healing Charlie and saving everyone on that opening day - but leading them to rescue, which appears not to be all it promised.
Jack's gift to heal is amplified on the island..miraculously saving Charlie when he was already dead. In doing so, did Jack course-correct Charlie's fate before Desmond?

What i'm suggesting is that these people had a natural gift which produced small miracules in the outside world..gifts which are capable of achieving even bigger miracules on this island..this generator of dreams and possibility.
I believe that Richard gave alot away when he recruited Juliet - he spoke to her as if they specifically were looking for someone who had achieved mircales in the field of fertility..it's almost as if finding someone with that prowess would make that persons prowess enhanced on the island, thus giving her gift a boost and making their chances of achieving mircales even better.
Jules doubts herself, but Alpert knows what she is capable of given the right time and place...

Juliet created life inside her sister - a real world mircale which the Others hoped would translate to "bigger things" on the island..

Unfortunately this has not been the case with Juliet - women still cannot successfully conceive on the island.
However, I believe that this is due to one of 2 reasons:
1. She's not committed enough. Ben once told Jack "I want you to want to help me". This indicates that a person, gifted or not, must be committed to the cause in order for the island to enhance their ability. Ben maes similar reference to committment in when he speas of his people not being prepared to make the final step in being one with the island.
Is Juliets lack of success due to her not wanting to be on the island?
2. Another possibility is that there is an even greater will (be it conscious or unconscious) blocking her own will to succeed. I have suggested in another theory that Ben could himself be the cause of the islands fertility problems - perhaps relating to his guilt ridden memory of his deceased mother, who died during giving birth to him. This would make sense..especially when we consider the "Magic box" is still in play.
Which brings me back to Locke - he is certainly committed and his will is as driven as anyones. The results can be seen not only in his continued ability to walk but in his communion with the island and the islands spirits (whispers, visions, dreams, Jacob):
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Amazingly, Locke is instantaneously healed upon landing on the island - his gift is amplified and his is able to walk once more..

However, Locke's gift appears not to only be limited to himself. As Ben knows, Locke's sheer presence can also heal others....

..A few days after being in a wheelchair, Locke heals Ben, who now only needs a walking stick to aid him - Benjamin himself admits it..proof indeed that Locke is special.

This would suggest to me that his natural gift is being supplemented by the island on a large scale - the island, which needs "help" appeared to recognise Locke's gift immediately. Locke himself believes that the island "summond" him..and Jack.
"The island brought us here"
So, the island would have known about each of their gifts beforehand.
If I was the island and I needed somesort of help, I would specifically target certain gifts which I needed to help me. This is exactly what Alpert seemed to do when recruiting Juliet - he wanted someone who had created break throughs in fertility. It didn't seem to matter that her break-throughs were only fully successful on a doormouse (at that stage) - all that seemed to be important was that she had "the gift".
To conclude, I believe that minor miracules in this world stand a good chance of being amplified on the island. The gifted are only limited by their own doubts or lack of belief. Alpert says it better:
"I want you to find your purpose".
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I hope to come back to this and update it as and when I watch the S4 episodes..perhaps this angle will be quashed, or maybe it can be strengthened.