As I have read some recent posts about the death of Locke (or lack there-of), I had an interesting thought that I think would be a pretty cool twist. What if later on this season, Christian shows up with some dialogue. Then later on, it is revealed that Christian's body is now being controlled by the real Locke. They would also reveal that MIB was using Christian's body when he was visiting Locke in Season 5. And when he changed and took the form of Locke, Locke's spirit somehow (farfetched, I know) took over the body of Christian.
There is however the issue of bodies. When Jack found Christian's casket, his body was not in it. This gave us the impression that something/someone physically used Christian's body to walk/talk on the island. However when MIB took Locke's form, it seems he became a replicate of Locke and didn't actually take the body. My only explaination to this would be Illana's comment that "(MIB) is stuck that way now". Maybe when he took Christian's form, it was sort of a temporary thing and he had to physically use the body. But with taking Locke's form, the only way he could do it was to become a duplicate of Locke's body, except in doing so, he had to stay that way permenantly. This may somehow involve "the loophole" as well.
I thought of this for a couple reasons:
1. Locke's comment in the FS in LA X, where he tells Jack that only his father's body is lost, not his spirit.
2. Locke told Jack in S5 that he saw Christian, which is what convinced Jack to come back to the island. I still think this is ultimately why Jack came back in the first place. It would be cool that after that, Locke actually becomes Christian.
I just think this would be a really awesome twist to the story. I picture Jack finally reuniting with him and being amazed that he is speaking to his dead father, but knowing he is talking to Locke. In a sense, Jack thought he let both his father down and Locke down in the past. Then Crocke would have taken Christian's memories and would know all about Jack's upbringing and understand why he was the way he was. He'd then tell Jack, "your father was very proud of you before he died", and Jack would know it is the truth because it was Locke reading his father's memories.
Okay I can keep going with my script writing, but I'll stop. Leave any comments you wish.