We have seen the end scenes of Lost at the start of Season 6 and I have an idea what they are by David Crosier

Why do I get the impression that Jack on the plane in 6.01 is quite literally the beginning of the end.

What if the way that this links with the on-island stuff is that these events take place not alongside as a fork in the road alternative to "The Incident" but actually as the eventual resolution to the arc on the island? I.e. We will rewind but not just yet.

Jack on the plane actually isn't another Jack in a parallel time line to on island Jack but the endgame reset?
Jack's slightly OTT fear of the plane crashing in this scene (total opposite of the 1st flight 815 when he reassures Rose, not the other way around) is less about his fear of flying/possibly dying but him actually knowing what's at stake if the plane doesn't go down/events are changed?

Matthew Fox's acting seems to be signposting a lot more going on below the surface and although I suppose that could mean anything I do think we're meant to pay a lot of attention to his slightly off behaviour. He looks for want of a better expression "Lost" at the start of the clip and then he realises he has to play his role as before.

I'm calling it here that Jack on the plane at the start of Season 6 is actually end of Season 6 Jack who retains the memories of all that has gone before/what we haven't seen yet.

"You can let go now" – Rose.
"Looks like we made it" – Jack
"We sure did" - Rose...Very last words of the entire series?

In a sense, if true, this scene would be flashback, flash forward, time travel, reset, alt and continuance of the storyline (What Happened Happened) all at once which is just about as clever as it gets!

Desmond being there and then seemingly disappearing could be that he is fulfilling some kind of Eloise Hawking role making sure things are happening as they should so that the best outcome for humanity is achieved.

Also, wasn't Damon asked (pre-Season 6) if we had already seen the end of Lost and he got quite agitated and said that it was a very good question. Maybe it was close to the truth and we would see the end before the end which would fit with what I'm guessing at.

Supporting clues are the island having been destroyed/being underwater and an injury on Jack’s neck being a surprise to him-ported over from end of season 6 action?

Jack knows the fate of humanity by the end of Season 6. He walks among us but he is not one of us!