I don't think it's gonna be Desmond. For the fact that he falls into that trio of people (Miles, Hurley, Desmond) who have special powers. Miles can see how people died, Hurley can talk to them after they're dead, and Desmond is the "constant" (time-travelling experiences, immune to electromagnetism - whatever). Walt has a special power as well, so maybe that's an opening to see his return before the show's over. IMO, the purpose of these characters has always been to help further the plot along, by bridging the gap between the normal reality in the flashbacks and such, and the supernatural stuff that the writers need us to know about. After all those secrets are explained, one-by-one, I think that the usefullness of the characters expires.
So, I see that trio (Miles-Desmond-Hurley) dying, Jack becomes the new Jacob, Locke stays as the new MIB, Kate and Sawyer become Adam and Eve in the cave, Juliet gets revealed as Jack's ex-wife in the sideways (so the producers can leave the show with a nice send-off to try and get people to watch V) and Ben and Richard do a "redemption" sacrifice play somehow in the finale to help Jack gain control of the island. If anyone gets off the island, maybe it's Claire, so she can finally be with Aaron again.