I think, after the characters travelled through the 4th dimension, they'll visit the 5th one as known as :
The dimension of choices and probabilities.
We've seen a lot of elements related to that in lost lately.
The theory surrounding the variable introduced by Daniel Faraday, says that we can change our destiny from our choices. (Changing the from the 5th dimension, the 4th dimension being unchangeable (constant))
Althought implicit, the question raised at the end of season 5, was what if flight 815 never crashed....we would be presented other set of events from this own moment. (5th dimension travel)
I think that what happenned at the end of the 5th season is that the characters changed the future by interacting on the 5th (variable) dimension rather than on the 4th (constant) one.
In my opinion, Hurley doesn't see dead people, he sees person from other strings of the 5th dimension which are created from different choices and results of probabilities. I think that's why we introduced Miles powers. It's been introduced as a hint about real nature of Hurley'S powers. Miles talks to dead people, Hurley talk to people that are still alive in other dimensions.
Well, I haven't much more time to put on this post, but I'm eager to hear what you think about that.