User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10750309-20131006103353

“Book five is when everything comes together, when they have to face the horde of giants that has arisen to take on the gods themselves, when they have to face the ultimate villain of the series, who is Gaea, the Earth Mother, who is not a happy lady,” says Riordan. link here

It somehow satisfies my theory

1) Gaea will rise

2) Gaea will raise the giants

It is said that they can't defeat Gaea, but maybe Ouranos can. So I'm guessing that Ouranos will be vital in the series. Book 5 might be all about raising Ouranos. What do you think?