I'd like to know what everyone thinks about this theory. I haven't really thought it all the way through yet, and there is still once aspect of it that doesn't really fit, but hear me out:
I'm theorizing that the FST is the end result of the Losties' experience on the island. The candidates are all brought to the island and given the chance (although they don't know this inherently) to correct whatever 'issues' they may have. Jacob explains this much when he explains what he is doing on the island. This part gets to be a little hazy, but I believe that either overcoming these issues or obstacles, OR 'joining' Jacob in this 'war' will ultimately lead to you getting what you wanted in the FST (I know 'getting what you wanted' sounds dumb, but I can't think of another way to put it).
Conversely, I believe that MiB's role in all of this is that he promises the individual something that he knows they want, but words his promises in a clever way wherein he does not deliver on his promise. For instance, he told Sayid that if he joined him, he would get to see the love of his life again. Welllll, in the FST, we see that this is indeed true. Sayid sees Nadia, but MiB never promised that he would be WITH her. Similarly, the MiB promised Claire that she would see Aaron again. Well, in the FST, Claire DID see Aaron, but it was on an ultrasound imaging machine. I'm theorizing that something is going to happen in the FST with Aaron, whether it be him dying in child birth, kidnapped from the orphanage after birth, or some other terrible occurance that will lead to Claire not having him.
I think that if you take this theory, you can make interesting cases for each of the characters depending on the camp they are in (FLocke vs. Jacob). For example, in the FST, Jack has a child and seems to be a good father. Hurley is the luckiest person in the world. Jin and Sun love each other. Etc. This is difficult however, because we still can't say 100% that we are sure which camp each character is ultimately going to end up in.
Ultimately, I think that there is a problem with the FST, and even though everyone who followed Jacob seems to have a better life, something tells me that it is not preferential. I believe that is where Desmond comes in... he is essentially deterring individuals from accepting this timeline. I'm not sure how they can rectify the situation, but I think that is where Des coems in.