Hello, first blog post on Lostpedia, so forgive me if I repeat a theory that has been posted before.
Is it possible, that with all of the mystery surrounding Christian Shephard and his relationship with the island, he had a similar role to Locke? I'm referring to Richard telling John that in order to save the island and bring the other losties back, he would have to die. Considering John in the coffin with Christian's shoes on the Ajira flight was supposed to mimic flight 815, is it possible that he was directly taking over the role Christian took? Both were dead upon landing on the island, and both are now seemingly roaming around at their own will, having a major influence on what happens on the island.
It seems pretty obvious to me that Christian has been to the island before his death and in order to get back and possibly "save the island", he would have to die, and this would be a great way to tie him to the island somehow, considering his status over the last 5 seasons.