1. Foreshadowing Death: In the season three episode "Expose" Locke tells Paulo "Nothing stays behind forever." Now, at the end of the episode people thought he was ferring to Nikki and Paulo and that they weren't going to stay buried and they weren't dead. However, this never happened. Now maybe, just maybe, he could have been foreshadowing three seasons later into his own death and resurrection. The producers said they ending they had planned since the Pilot was still the same so it's possible they had planned Locke to die but come back.
2. Walt's Dream: Walt says in "The Life and Death of Jeremy Bentham" that his dream was about him wearing a suit and people around him trying to hurt him. Everyone thought he was talking about the Ajira people but we know that other than Ceaser, no one was trying to hurt him. Maybe Locke will come back (still wearing the suit he was buried in) and someone will be trying to kill him.
I'm trying to think of more theories but I have a strong feeling Locke is not dead and will be back before the end. I'll post later with some more theories.