Board Thread:General Discussion/@comment-6036560-20131216221352/@comment-5235494-20131231184651

Here's my guess:

In "Red Handed", Granny made a direct reference to the Underworld - ("I'll sleep with the trolls in the Underworld", or something to that effect, when she was chastizing the hunters of her village for going after the wolf)

By Season 2, Cora mentioned to Aurora that after Prince Phillip's soul was sucked out by the wraith, it was sent to a different world, where it could be retrieved.

Since Rumple killed himself along with Pan, maybe their souls went to the Underworld. It would be cool to see all of the characters who died (i.e - Snow Whites mother, Regina's father, Cora, etc.) again, which could lead to several characters having tearful reunions and also give insight to how characters changed after they died. Plus, Hades could easily be introduced into the plot, as Emma/Regina might make a deal with him to retrive Rumple's soul (and possibly the other souls of those they lost).