Aaron is Jacob
In a recent episode, the MIB (as Locke) runs into a little blonde boy in the woods who says, “You know the rules, you can’t kill him.” I have no idea who or what he meant by that, but I’m pretty sure that the person Jacob discussed bringing to the island is Aaron. It’s been set up since the finale of season 5 that Jacob and that the MIB are the two ultimate powers on the island and they’re basically polar opposites from everything to their attitude on humanity to the color of the clothing choose to wear. It’s also been made abundantly clear that the MIB can appear to those on the island as people who have died. However, it seems that his current disguise as Locke is much more material than his other apparitions throughout the show.
I’m going to say then that instead of just being an illusion, he actually has fused with Locke, although it is evident that Locke may still exist within the MIB’s takeover. His response to the child in the forest was and emphatic, “Don’t tell me what I can’t do” after all. Anyway, in order for him to fuse with Locke and become an actual person, he needed to find his loophole that he and Jacob mentioned at the end of the fifth season.
It has also been claimed (with reasonable evidence) that Jacob can appear through animals (vincent, Kate’s horse). However, (and this is really the main point), if the MIB can appear through those who are dead on the island, and Jacob and the MIB are polar opposites, then it would seem to make sense that Jacob can appear through those who are born on the island. However, he also had to find a loophole, because those who get pregnant on the island get an illness (we’ll get back to that later) and die. So, just as convincing Ben to kill Locke so that he wouldn’t have to do it himself was the MIB’s loophole, Claire getting pregnant off the island and then giving birth on the island was Jacob’s.
Now back to the illness, the illness is probably similar to what has happened recently to Sayid and Claire, which seems to be caused by the MIB. It’s possible that MIB has prevented Jacob from being fused with another person by getting pregnant women sick with the unidentifiable “illness.”
Also, Jacob’s sole concern has always been protecting the island. If someone needs to protect the island and he hadn’t found a replacement for himself yet, then why didn’t Jacob fight back when Ben attacked him? Because he knew he was coming back to the island as Aaron just as MIB was born again through Locke.