Right this theory isn't going to be too long, but i just wanted to point out that the MIB seems to repeat how much of an idiot John Locke was for believing he had a purpose, its almost as if he is desperate for them to believe it. Jack said in the last recruit "this guy wants us to leave so badly, maybe he's afraid what will happen if we stay" This was a key part because he's right, John Locke scared the MIB because he knew there was a bigger purpose, he was a man of faith and a possible candidate. This is exactly the same as what we are seeing about desmond, Locke asked him if he was afraid and Desmond was not, this scared MIB and it has become apparent that anything that scares him He tries to Kill another example of this would be Mr Eko. Eko was also a man of faith and he did not fear judgement The MIB thrives on the fear of others and if he senses a threat he will dispose of it by whatever means n! ecessary. So if Jack is a candidate and we've noticed that he has become a man of faith or belief in recent times, then maybe that's why he will be MIB's next biggest worry.
One final note aswell, i wouldnt entirely rule out Desmond as a candidate, think about it Charles told him he would need to make a sacrifice and what would be more of a sacrifice then to give everyone else a chance at another life where things could be different.
Please give me some feedback guys :)