I know the subject of Jacob is probably talked about frequently, but I don't think its a mysery that can possibly get old. My whole purpose here is to just through out some of my personal ideas on the subject.
I have heard some people speculate this person or that person will end up being Jacob. I am critical of it being any Lostie/ or other character now present simply because of the fact that he has not even been casted yet. Also the fact that Richard recognized the name Jacob when Locke tried to convince him he was his leader in the future.
However if Jacob is a character we know now and love I have it down to one person. The one that makes the most sense to me is Desmond. Simply because of the timeline issue im assuming he will have to enter a boat race on this and this date to get back. If Penny and his son Charlie are dead Desmond will be a wreck and possibly cause him to be the mysterious sometimes disturbed looking creature we see. If sent back far enough he may even possibly end up on the black rock or at least near its arrival. However my theory would also entail Desmond being trapped in the cabin by Ben some how thus why he called out to Locke and Hurley (they were friends on the island) with "help me."