Not sure if this theory has been put forth yet, as I generally try to stay away from reading theories.
I was just thinking about the trajectory of season 6-- specifically that there must be a point to the alternate/sideways timeline, as roughly half of the first 3 hrs of season 6 have been devoted to it. I'm not sure yet what that point is, but doesn't it stand to reason that Kate, Jack, Sawyer, possibly Hurley, and possibly a number of the other 815 passengers, are going to somehow end up on the island in the alternate/sideways timeline because Jacob has summoned them there?
Is it possible that there could be another plane ride (and then plane crash) in the alt timeline which brings the core group of characters together again?
I'm looking forward to seeing, in the coming episodes, how the 815 passengers are all joined together in the alternate timeline. It seems that the groundwork has already been laid for this to occur-- Jack gave John Locke his business card, now Kate and Claire have become involved with one and other.. who knows how Hurley and Sawyer are going to tie in, but surely they will somehow.