Wowza, with major characters falling right and left, those flashsideways had better be a virtual reality or purgatory style scenario. It ain't right to kill off characters when their stories are left so unfinished.
At the rate they're going, I think all the characters will be dead by the end of the series finale - but we've already seen how the story really ends.
Something about the flash-sideways determines which of the candidates takes Jacob's place. My brain is too overloaded to figure this out right now, but the whole thing strikes me as one big gigantic test.