One comment Jack made in this episode struck me. He told Sun not to worry because he was going to help her find Jin and, "...get you as far away from the island as I can." Not an exact quote, but I know he specifically said, "you" not "us."

It seems that Jack may already be gearing up to take Jacob's role...?

This was obvious once we saw the cave. Also, remember in "Through the Looking Glass" when Ben says to Jack, "what do you have to get back to?"

Jack has nothing off-island.

I think that's a good observation and a very plausible theory. I especially think the backup in the form of Ben's dialogue helps it out.

Oddly enough when Jack offered his hand to Sun, I immediately thought that Jack has accepted that he is the replacement for Jacob.
He could be wrong of course, but he thinks he is at this point.

This was obvious once we saw the cave. Also, remember in "Through the Looking Glass" when Ben says to Jack, "what do you have to get back to?"

Jack has nothing off-island.

The "What do you have to get back to?" made me realize everyone on that island is going to die. We heard from Juliet that the bomb worked in '77. We see the flash sideways of what is going on off island. If they were to get on a plane and fly away, there would be two of each of them in the alt/real world. Doubt they'll go the route of 2 Jacks, 2 Kates, etc... So either everyone lives happily ever after on the island, or they're gonna die. No one is getting off that island. I'm betting on dead losties.

One comment Jack made in this episode struck me. He told Sun not to worry because he was going to help her find Jin and, "...get you as far away from the island as I can." Not an exact quote, but I know he specifically said, "you" not "us."

It seems that Jack may already be gearing up to take Jacob's role...?

I'm betting Jack gets Sun on the plane...... but here runs Jin at the last possible moment. Or, Jin is on the plane and up runs Jack with Sun. Whichever. Anyway, there is no more room on the gerryrigged plane. Kate (on the plane seated next to Clare) realizes that there is no life for her back home. That the only two people she loves are Aaron and..... gasp....suddenly realizing, Jack. (Is Sawyer dead??) Kate makes eye contact with Jack and then jumps off the plane at the last moment. Jack practically throws Sun or Jin onto the plane. They kiss (Sun and Jin, Kate and Jack). Aha! The viewers exclaim, "Jack and Kate are Adam and Eve". Epiloque: the years pass, Kate/Eve and Jack/Adam parent a set of twins. Cane and Abel who will later become alias Jacob and MIB. Please refer to the Bible for the entire story....it makes sense. LOST has come full circle. The End. Hoorah. Viewers are going "Holy Sheet" and running to their keyboards to post on The Fuselage.

Oddly enough when Jack offered his hand to Sun, I immediately thought that Jack has accepted that he is the replacement for Jacob.
He could be wrong of course, but he thinks he is at this point.

I don't think Jack thinks he's the replacement at all. Keep in mind, he has not been privy to any of the conversations between Jacob and Hurley or Jacob and the MIB with regard to rules and that Jack has something important to do.

Jack simply accepts that he's on the island for a purpose and that he might be the candidate. I haven't seen anything to indicate that Jack thinks HE'S the candidate. If anything, we the audience, are seeing things in Jack that makes us wonder if he might be the candidate.

What reason has got to home?