I think somewhere near the end of the show some of the main characters (maybe candidates) who remain alive will get a chance to become kids again (like baby kate, Jack, Sawyer, etc) and then be sent back to the past via the donkey wheel. Then they'll grow up again having all the island experiences vaguely in the back of their minds. The alternate timeline actually shows them when they grow up to be adults again and end up in flight 815 once again. They get a second chance to live different lives just as Locke tells Shannon in season 1: "Everyone gets a second chance to start a new life on this island" and the result is what we see... Ben becomes a teacher, Hurley is lucky and etc. The ones who have died in the first time round may not die in this second time round because of the different interactions that they may have with the time-travelled characters.
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