That made me think it could not have been Sawyer because regardless of what could happen between them in the end, they had an affair so Kate would feel sad about his death. So, it has to be someone Kate does not care about but Jack does. Juliette, maybe?
Here's my take: The person in the coffin is one of the Oceanic Six. Jack's flash-forward to his shell of a person with that awful beard is in the future in relation to when he sees Hugo in the gym at his mental hospital. He's been following the fates of the other five all those years and learns one has died. So the person in the coffin can't be Ben (like I had thought last year), because he was not on the flight.
My guess: The Oceanic Six are Hugo (Hurley), Jack, Kate (as we know), Claire, her baby, and Locke. Locke is in the casket. Locke is the "He" Jack is referring to when he pleads with Kate that "He wants us to go back to the island." And the "he" waiting for Kate is the fiance she took off on right before she went on the lam and ultimately ended up on the flight - Nathan Fillon is the actor. Sadly, this means that my beloved Sawyer is either still on the island or dead.

Then again, keep in mind that it's said there are three more seasons planned. What if all this transpires just this season, and that next season, the remaining five (or at least Jack) go back to the island to save (or live with) the others?