The Substitution by DoYouLikeEkoStick?

For this theory you need to know what the word SUBSTITUTION means.

Substitute: A person or thing that takes or can take the place of another.

A person that can take the place of another.

Now back to the theory, we all know on island, theres only one person capable of taking the place of another, because they have done it before, and that person, is the Man In Black/The Smoke Monster. The smoke monster has taken the image of another, but this isnt the substitution. Lockes image NEEDS to be substituted for another by the man in black, for many people are now aware that the man who looks like Locke, isn't. That wouldn't be the most cunning of disguises anymore.

So who can be swapped for lockes image?
Since lockes image is no longer needed because it served its purpose, he needs someone who is closer to the other people on the island.

And who would be the perfect 'candidate' for this?
James Ford. Sawyer.

It seems now Kate is going to go off on her mission to find Claire, and Sawyer is all alone. The smoke monster is also setting out to a place he has in mind it seems. This is Sawyer.
Alone sawyer will be confronted by the smoke monster, and in my opinion, he will be killed, and his form taken, by the man in black.

Fake sawyer could acomplish a lot more than fake locke now.

This is only a theory.
But seriously, don't bet on sawyer living much longer...