While this would be a fine bookend, I know people who have been predicting this at the beginning of the season (not Claire, but the same scene, and always with at least Jack), so I'm guessing it won't happen that way. That's a good point you raise about Kate too actually. Homer, I'm guessing, since they saved the reveal for the finale, that Jack's wife is going to be Juliet, even though that doesn't really fit with how they've treated the sideways timeline so far. With the exception of Sawyer and Miles, none of the characters knew each other anymore than they did in the original timeline. So for Jack and Juliet to have married and raise a child is a big swing. Not to mention, in the sideways so far, relationships have largely repeated, so I would believe Jack would have married Julie Bowen again. However, Juliet wouldn't have spent years on the island either, so, who knows. Anyway, my guess is that Juliet will be at the concert, and possibly Sawyer ends up there trying to find Kate and meets Juliet and bam, we all get visions of the other life, as the other characters who experienced love did.