Since it's a quiet week with no new Lost, I thought I might go through some of my old theories postings to see if we've learned anything new since I posted them. Keep in mind, most of these were posted between Seasons 3-4, so we've come a long way since then.
My very first Theory Question regarded Richard Malkin, who was one of the most puzzling aspects of the entire first season because more than anything else, his insistence (and later admittance that he was a fraud) strongly implied that someone knew that Flight 815 would crash and specifically hired him to get Claire on that plane.
This is especially interesting now that it seems Claire (and Aaron's) role seem to have been diminished by the fact that neither of them went back in time. Now one could argue since Aaron is one of the Oceanic Six his presence helped get Kate, Jack and the Others go back in time, but there certainly wouldn't be any time-loop problems if a pregnant Claire never got onto the plane.
That being said, someone still must have hired Richard Malkin to get Claire onto that specific flight. One of my initial thoughts was that Christian, before he died, without any knowledge that Flight 815 was going to crash, hired Malkin to put Claire on the same flight as her brother and that the adoptive family waiting for her was going to be Jack's mom. Thus Claire's presence on Flight 815 was kind of an accident - Christian had no idea that the Flight was going to crash, he just wanted her on the same flight as Jack. This also kinda preserves the "you must raise this child yourself" theme. Once in the states, Claire could live with her half-brother and his family and raise Aaron herself.
But the time travel changes everything. Now we know, assuming they survived the decades, that there are several people in 2004 with prior knowledge of the crash: Jack, Kate, Hurley, Sawyer, Sayid, Miles, Daniel and Juliet, plus anyone they might have told over the years. Any of them could have arranged for Claire to be on that flight.
Now why would they if they knew the plane was going to crash? Well, my guess would be that they essentially become course-correctors, making sure everyone who's supposed to be on Flight 815 gets on there. Claire certainly needed a good deal of persuasion to get on that plane; certainly wouldn't have herself without Malkin's influence. And they might not have to do it directly, just as long as they leave specific instructions on what has to happen and when.
That's my updated guess for now - Malkin was one of the first things that fascinated me about Lost. I have no doubts we'll get an answer to this question before the end of the show.