Assuming he lives, I think he will appear like the cavalry at a pivotal moment and instead of helping either the Losties or the Others, will throw an entirely new monkey wrench into the mix by somehow ruining any chance for ANYONE to get off the island, doing whatever it takes to make sure he (Locke) is able to stay and the outside world will not interfere. He is on his own journey now.
These are my thoughts too, but you said 'em much better than I would have!