I don't claim to be an expert, for as we all know, anything on LOST is possible. I do, however, hear of some pretty "interesting" theories from other fans that I don't understand. If you share one of these theories, please provide some sort of explanation.
Aaron is Jacob. I don't believe this for the simple fact that Aaron's name is not Jacob.
MIB and Jacob are the same person. The whole time travel aspect of the show has made this possible, but highly unlikely. If they were the same person, then they would have the same goals, so why would they be rivals if they are the same person? Why do they look completely different? Further, time travel has little to do with what's happening in this season. I don't think we'll find that time travel will explain much of any remaining mysteries.
MIB is Jack's son in the alternate reality. I honestly feel like people are just making this assumption because Jack's son has black hair. Jack's son has black hair because Jack has black hair. Genetics. And again, I don't think time travel will play much of a role for the remainder of the series.
Here's what I think is the history of Jacob and the MIB. I think the both of them were brought to the island just like their predecessors and their predecessor's predecessors. I feel like the candidate process is one that has taken place many times before (think of the path of The One in The Matrix Trilogy…something like that). I feel like the leader of the island and the protector of the island play a role in choosing candidates and bringing them to the island. In this case the leader and protector are Jacob and MIB (respectively).
MIB is sick of this process and no longer wants to fulfil the obligations attached to his role as protector of the island. He has gone rogue. Jacob brought the Black Rock to the island (the same way he brought Jack and the gang) because some of them were to be candidates. The MIB then tried to relive himself of his obligations because he did not think it was fair that people be "chosen" against their will (we see hints of this in the opening scene for The Incident).
Jacob, having knowledge of MIB's eventual rebellion, made a deal with Richard (a slave on the Black Rock) that promised Richard eternal life and freedom if he helped to contain MIB. As punishment for his rebellion, the MIB became formless black smoke and only able to take on the shape of people who have died in great emotional anguish (Christian Shepard and John Locke); all while still needing to fulfil his obligations as protector and guardian of the island.
MIB found a way to gain his freedom according to a loop hole in the rules: Kill the leader of the island so that a new one may be appointed. The newly appointed leader may choose to keep MIB as protector or appoint someone else. My guess is that he is deceiving the people he is "recruiting" so that one of them can take his place and NEVER go home as he has promised them. In the end I think Jack will be the new Jacob and either Sawyer or Sayid will become the new MIB (I'm leaning towards Sawyer at this point).