This may have already been discussed, and it may be blatantly obvious, but I have a theory as to why the Oceanic Six had to return to the Island. We have seen that Eloise Hawking is some sort of "temporal policeman," when she tells Desmond in his altered flashback that he couldn't buy the ring because he couldn't marry Penny yet because the Island needed him. So Hawking seems to have knowledge of the future, and probably has extensive knowledge of the past. The reason why she took such an active part in bringing the Oceanic Six back to Island was because she knew the universe would course correct itself if they were not brought back to the Island. The Oceanic Six coming back to Island had always happened- because they came back to the 1970s and were always a part of the past of the Island. If Hawking, Ben, Locke, and Widmore hadn't tried to bring them back to the Island, the Oceanic Six would have eventually come back no matter what because they had always existed in the time of the Dharma Initiative, evidenced by the Dharma recruit photos that Christian showed Sun. Hawking knew they had been on the Island in the 1970s, and since she seems to have knowledge of the future, she knew when they had to come back so they could flash back in time. It seems to be that the each of them has a purpose for the Island. Kate's purpose seemed to be to save Ben's life when he was a kid. Sun has a long journey ahead of her, what exactly that is we don't know. Sayid's purpose was to "kill" Ben so he could then be healed by the Others. Jack and Hurley's purposes are yet to be revealed, but I would assume Jack has a fairly large purpose coming up soon in the final 4 hours of the show.