So I was thinking back to the episode "The Man Behind The Curtain"; remember the guy everyone thought was Jacob in Jacob's cabin?
What if this is actually The Man in Black? It would make sense, seeing as he says to John Locke, "Help me..." He wants to get off the island. The man in the chair is very aged, when we know now that Jacob has retained his youthful appearance, while the Man In Black is older. It makes sense to me. Complete sense.