Now that it's clear that Locke is dead, it removes the possibility that he will replace Jacob in the long term. It may be that removing both Jacob and his rival is the endgame. But if not, if there must be balance, the logical replacement for Locke may in fact be Jack. In the wake of Locke's death, Jack has been acting more and more like someone locked into a destined role. If he can let go of the notion that he must control everything in the world (in other words, if he embraces the predestined nature of time in the "Lost" universe), then he could combine his former leadership role with Locke's spiritual psychology. It would be a great payoff to his long journey towards redemption.
But it's not likely to be an easy road, since it's most probable that Miles was correct: Jack's plan will probably create the Incident, not prevent it. More than that, it's likely that the Incident will toss the Oceanic Tribe back into their proper time and cause the conception issues that plagued the Others in earlier seasons. That travel back into 2007 would essentially kill Juliet (who wasn't going to survive much longer anyway), who would "reappear" in the middle of the ruins of the Swan Station. Sayid's situation is also grave, but perhaps not so much so, based on the fact that Juliet was the only one with a flashback that didn't involve Jacob.
But otherwise, it looks like everything would make the transition more or less intact. That might even apply to Rose, Bernard, and Vincent, who had at least found some measure of happiness on their own. If the rest of the Oceanic Tribe make it back to 2007 as a result of the Incident (and are then presumed dead by Richard because they were at "ground zero"), then shouldn't everyone caught out of time do the same? (This would appear to be the meaning of Jacob's final words: the Oceanic survivors are returning to their proper time.)
Most of what happened in 1977 was so predictable, right down to the fistfight between Jack and Sawyer, that it almost took away from the episode. Only Juliet's odd behavior broke it out of that mold. But even that was fairly consistent; Juliet has been making decisions based on the knowledge that her time with Sawyer was over as soon as Kate returned on the scene. Juliet's actions have contained such a sense of inevitability that it was practically self-fulfilling prophecy.
That renders her character arc rather tragic, but that fits the arc of just about every other character to this point. Even Ben's story is tragic. He doesn't even realize how long he's been manipulated, going back to when he was a child (it's a fair bet that his vision of his mother was Jacob's rival). If Jacob's rival represents Anubis, a god of the underworld, perhaps Ben was saved through the power of Jacob's rival. That might be why Ben was never allowed to see Jacob himself.
On the other hand, it's entirely possible that Jacob knew that Ben was going to kill him, and that this moment was coming. And perhaps that is why it was important to show that most of the few remaining Oceanic Tribe members had been "chosen" by Jacob at some point in their lives. Either Jacob was sure that he would be replaced by one of them, or the loss of his physical form might put him on an even playing field with his rival.
Regardless of how much of this speculation is proven wrong (and some of it inevitably will be), the stage is certainly set for the final season. Using the familiar white-out of a time flash for the end title may be more than just creative license. If the past five seasons have been dominated by the machinations of Jacob's dark rival, then this may be a sign that the final season will render matters back into balance.