In the last episodes of season 3, Desmond keeps seeing visions of Charlie dying. What if the same thing could happen again in the sideways? What if in fact, this whole loophole isn't the first time it's happened.
I thought of this when I was reading about the time difference on the two time lines. If Desmond is seeing visions of his time on the island, then he could have visions of Charlie dying again (only it's probably suicidal, but if it weren't). So how come Desmond saw those visions in the first place? Could it be they've looped into similar positions and not remembered it before? Like, it's all too coincidental that these specific people met on the island.
Another thing, Jack has the most connections with all of our characters. So I think he is the most important. It's like, if Jacob is in the light house looking over Jack's life he must look at him a lot. Think about it like a news website that has external links to other sources: You might go off on a tangent to another website, but you always come back to the main website. If Jacob's been watching Jack, then he saw Shannon's dad dying in the room where Jack saved Sarah's life. And that's how Jacob started looking at Shannon and Boone. While watching Jack he also saw his father, and that might have led him all the way to Australia and seen Claire. Through Jack's father you get Ana Lucia too. I have a feeling there are other connections too?