I was just asking myself that question when I was rewatching the episodes of the previous seasons. I think that all of the characters have a destiny and a purpose, maybe the purpose of John Locke was to bring everybody back and make them believe that the Island has something prepared for everyone of them. But the story of Locke, is a story of a man, who had a terrible life, and experienced a miracle, but he (in my opinion) is not that special... I mean, I think he always thought of himself as the chosen one, but we all saw Richard Alpert asking Jack about John Locke, and telling him that Locke didn't seem special at all...but when he met Benjamin Linus in a very young age, he saw something in him, and told him to be patient, and he became the leader of the Others. Locke, was only an old man chosen to "persuade" the Oceanic 815 survivors of believing in the Island. But he served his purpose in the general story, and died. And now his body is being used by someone or something really scary. I believe we saw the last of John Locke, and I hope he doesn't get resurrected. The Island is done with him, and so am I.