If this has been posted before I apologise I do not read every theory.
I have thought for a long time the Island was a natural satellite or moon if you prefer. However instead of the way the moon in the sky travels through space whilst orbiting the earth. I believe this moon orbits the earth probably on a strange trajectory whilst travelling through time as opposed to space.
Evidence if you remember back in season 5 when the oceanic six are trying to return Hawking says they need to know not where but when the Island is in time.
Also I have a feeling that if something were to happen to the Island then this would have a major consequence for Earth causing a major natural disaster, which will wipe out life. I think this is probably what will happen in the flash-sideways. We see the Island buried in the ocean but I think that at a fixed point in time the consequence of it being destroyed will kill everyone.
Because of this I think the memories of our losties and the sideways losties will start merging maybe even a few Desmond style conscience jumps, which would be cool.
Anyway that my thinking I would wager I am horribly wrong but that the beauty of lost.