User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5651660-20121218212918

Here's what I think:

We're gonna hear more about Vestal Virgins, as noted in the beginning of MoA

The Hunters of Artemis will make an appearance

Calypso will appear, as promised by Rick Riordan, in either HoH or the fifth book

Percy and Annabeth will both die sometime in the book, and then come out on the other side of the doors of death.