Oh, man, I was laying in bed and just had this realization.
What if everything I said above was true ... except it was Desmond who had to make the choice instead of Jack?
Think about it. He's immune to the properties of the island, he can shift from one timeline to the next ...
A bunch of us have said it seems odd that Jack can "choose" to take over protecting the island. What if he's not the one that's meant to be the new Jacob? What if Desmond is?
I say this because I envisioned all the castaways dying on the island and Desmond going into the lightsource. He then can see both worlds, the on-island world where he must protect the island at all costs and the off-island world where the castaways lived and everything is right.
Here's the kicker. The one thing he cares about the most (Penny) he can't have in the sideways world. Why? Because she's married to JACK!! (Cue dramatic music).
Forgive me if this is just a 5 a.m. rambling of a guy who needs sleep but if this comes true, I will be excited that I hit it on the head, but will be sad we're missing out on the only real romance of this series.