I was thinking about it and I may have a better answer as to the "loophole" that the man in black mentioned to Jacob in the openning scene, who I will call Johnny Cash, because I dislike the theory that he is Esau.
First, my theory starts with one really big presumption, that Jacob is the one who sent the castaways back in time. These people who are in a time "loop" or at least a "fold" were selected by Jacob and sent to a specific time - 1977. The castaways that I'm speaking of the are the Oceanic 6 minus Sun that were on Ajira 316 and blipped to 1977 before the plane landed. I am unsure about the castaways who got to the 70's that were already on the island (Sawyer and crew) as they may or may not have been sent by Mr. Cash rather than Jacob. The important point is that the Ajira folks who ended up in '77 got there in a very different way than the rest of the castaways. They were selected out of a bunch of people on the plane to be there (Is it me or does that sound reminiscent of the opening of the god-awful Left Behind series?).
Ok, so to the main theory. I believe Locke's death was the "loophole." First let me start with the time skips. It appears that a rule to the time travel is that dead people or dead bodies cannot be sent through time. This was revealed in "This Place is Death" when Charlotte did not "move" with Daniel and Sawyer after she died. Her body stayed in the time in which it died. That was why Mr. Cash started the plan to have Locke die before returning to the island.
This theory would seem to indicate that Jacob could have been acting through Ghost Christian to send Locke back by moving the island. Jacob needed Locke to bring the O6 back to win the "war" - whatever that is. This was the "loop" in which Mr. Cash needed to create a "hole." So during the time jump, Mr. Cash planted idea that Locke needed to die to bring them back. From what I saw, Locke's death was totally unnecessary to bring the O6 back, as we saw Jack was freaked out and already trying to get back to the island when he heard of Locke's death. (remember he read about Bentham's death while taking planes to Australia hoping they would crash). In fact Locke's death pushed Jack to attempt suicide and not return to the island. And Locke's death wasn't much of an issue when convincing the others to return to the island.
Locke's death prevented himself from being sent to the past (ala Charlotte) with the other castaways, and thus kept him out of the Dharma photos so that Mr. Cash could impersonate Locke and get Ben to murder Jacob. And thus, Mr. Cash could then push Jacob into a "Burnin' Ring of Fire."
I believe Jacob, however, in a very backgammon like way, "course corrected," and reversed the game board, so to speak, and moved the once suicidal Jack into the Locke, man-of-faith, role. Locke was supposed to detonate jughead at the swan station. Jack did what Locke was intended to do by getting jughead and dropping it in the Swan. This, was the big surprise to Mr. Cash at the end of the Finale. Cash thought he had stopped the return of the castaways from the past by preventing Locke from being there to detonate Jughead. Jacob's final words shocked him because it told him that Jacob had found another way to bring them back. In fact, Jacob have "converted" Jack, the greatest skeptic and antagonist towards Locke's man-of-faith, and used him to bring about the return. Thus, Jacob's last words - "They're coming" - certainly referred to the '77 castaways shocked Mr. Cash because Jacob had taken one of the biggest doubters and mess makers on the island, or as Mr. Cash would ! call him - a corrupter, and made him the a man of faith in the island. Turning Jack into a man of faith was something Mr. Cash didn't believe was possible.
First, my theory starts with one really big presumption, that Jacob is the one who sent the castaways back in time. These people who are in a time "loop" or at least a "fold" were selected by Jacob and sent to a specific time - 1977. The castaways that I'm speaking of the are the Oceanic 6 minus Sun that were on Ajira 316 and blipped to 1977 before the plane landed. I am unsure about the castaways who got to the 70's that were already on the island (Sawyer and crew) as they may or may not have been sent by Mr. Cash rather than Jacob. The important point is that the Ajira folks who ended up in '77 got there in a very different way than the rest of the castaways. They were selected out of a bunch of people on the plane to be there (Is it me or does that sound reminiscent of the opening of the god-awful Left Behind series?).
Ok, so to the main theory. I believe Locke's death was the "loophole." First let me start with the time skips. It appears that a rule to the time travel is that dead people or dead bodies cannot be sent through time. This was revealed in "This Place is Death" when Charlotte did not "move" with Daniel and Sawyer after she died. Her body stayed in the time in which it died. That was why Mr. Cash started the plan to have Locke die before returning to the island.
This theory would seem to indicate that Jacob could have been acting through Ghost Christian to send Locke back by moving the island. Jacob needed Locke to bring the O6 back to win the "war" - whatever that is. This was the "loop" in which Mr. Cash needed to create a "hole." So during the time jump, Mr. Cash planted idea that Locke needed to die to bring them back. From what I saw, Locke's death was totally unnecessary to bring the O6 back, as we saw Jack was freaked out and already trying to get back to the island when he heard of Locke's death. (remember he read about Bentham's death while taking planes to Australia hoping they would crash). In fact Locke's death pushed Jack to attempt suicide and not return to the island. And Locke's death wasn't much of an issue when convincing the others to return to the island.
Locke's death prevented himself from being sent to the past (ala Charlotte) with the other castaways, and thus kept him out of the Dharma photos so that Mr. Cash could impersonate Locke and get Ben to murder Jacob. And thus, Mr. Cash could then push Jacob into a "Burnin' Ring of Fire."
I believe Jacob, however, in a very backgammon like way, "course corrected," and reversed the game board, so to speak, and moved the once suicidal Jack into the Locke, man-of-faith, role. Locke was supposed to detonate jughead at the swan station. Jack did what Locke was intended to do by getting jughead and dropping it in the Swan. This, was the big surprise to Mr. Cash at the end of the Finale. Cash thought he had stopped the return of the castaways from the past by preventing Locke from being there to detonate Jughead. Jacob's final words shocked him because it told him that Jacob had found another way to bring them back. In fact, Jacob have "converted" Jack, the greatest skeptic and antagonist towards Locke's man-of-faith, and used him to bring about the return. Thus, Jacob's last words - "They're coming" - certainly referred to the '77 castaways shocked Mr. Cash because Jacob had taken one of the biggest doubters and mess makers on the island, or as Mr. Cash would ! call him - a corrupter, and made him the a man of faith in the island. Turning Jack into a man of faith was something Mr. Cash didn't believe was possible.
