This is my first theory,so I apologize if any of this is going over old ground,but I started thinking about the main characters that are left and all of them have serious father issues.
jack,sawyer,locke and kates fathers are all dead and they all had seriously flawed relationships when they were alive.sun's father is a thouroughly nasty man and jin pretended his father was dead because he was ashamed of him.hurleys father ran out on him when he was a child.I cant recall sayids father but I'm guessing it wasn't the most balanced childhood,seeing as he turned into a torturer!
I am wondering if these relationships have anything to do with them being the candidates,maybe as jacob being some sort of a father figure or guiding them to perhaps let go of the past with their fathers that has really affected all of them very strongly.
we've seen revenge-sawyer,kate,sun,greed-hurley ,doubt-jack,locke.
and the father theme seems to run strongly throughout the series.
I'm not sure if this is relevant or not or how it connects to all the other mind bending questions that are yet unanswered but thought it may make a bit of sense.
be gentle!!!