I've been thinking about Desmond's interaction with Jack on the plane during the flash-sideways timeline. Desmond sits next to Jack and they have a conversation. Jack later leaves his seat and returns to find Desmond gone. He asks Rose and Bernard if they had seen him but they hadn't as they were sleeping.
It is strange that Desmond would suddenly be sitting next to Jack in a seat that was previously unoccupied. And it is also unusual (at least I think so) for a plane passenger to get up mid-flight, leave the seat they are assigned to, and sit in another chair for a few minutes.
I think that Desmond is playing the role of Jacob to the island in the flash-sideways timeline. Desmond somehow projected himself onto the plane so that he could have an interaction with Jack. In the conversation they had Desmond might have been judging Jack to see if he had grown in his experience on the island.
I don't have much else to offer right now but I'd like to hear what anyone else thinks about this idea.