Ok, So this theory was one that i have put together from another theory and some of my own thinking. I read someone posting that they believe Desmond was brought back to the island by widmore to be sent back in time and change the past.
In season 5, when the losties are traveling through time, Faraday went and found Desmond back in time. He said to him "Go and find my mother, Eloise Hawking." Desmond then woke up 30 years later and it was like a new memory had been formed. This shows that Desmond can change the past/future.
I think that Desmond has been brought here to change the past, and that change is going to be sinking the island. This is going to involve the Losties who were living in Dharmaville, like Jack Sawyer and Juliet, and will change them from exploding Jughead where they did to doing something else.
After this, the alt-timeline that we have seen now will happen.