It is to be speculated that Melisandre might have helped Jon Snow to bring back from the dead. The trailer revels that Melisandre puts her hands on Jon Snow’s head, which means that she might try to save him. It has also been seen that whenever Melisandre takes off her clothes she is about to perform a magic and that’s what is happening here too.
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So looks like she is trying to resurrect Jon Snow. The trailer also shows Ser Davos protecting Jon Snow’s body with a Longclaw and make sure that nobody come close of his body from the Night’s Watch. Night Watch though try to break the door as Ser Allister Throne can be seen standing behind the soldiers.