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Oh yes... it's fairly obvious the concert is only the beginning of whatever the sideways characters will do in the finale.

For example, the concert is one the same night, so it seems like Jack can only fix Locke later, in parallel to the resolution on the Island where he fixes the Smocke situation. There may be days, weeks or more of story yet to be told in the sideways.

I'm pretty sure it's not only about Desmond making them see their other lives, it's also about figuring out together what they must do about that, and the key to that, the one with the answers once he remembers the Island events, would seem to be sideways Jack since he's the guardian. Sideways Daniel, Eloise, Widmore etc. also have yet to get involved again (Will Eloise be the sponsor/organizer of the concert? I guess it's possible)

The writers have done a very good job at building up to "fake endings", like Desmond reuniting everyone, or the Ben-Widmore confrontation, the blowing of the plane etc. That looked for a while to be "the goals" the finale would deal with, but as the episodes unfolded that turned more and more into merely the beginning on the end. As a result, they haven't left us with more than a few clues about the action/content of the finale.

There was a really weird promo last week for the finale. I had dismissed all the new footage shown as some sort of joke, but perhaps I was wrong (I remember not much, but there was a clip of Locke as a WWII era general, and Dr. Chang was in there too).