User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-108.3.134.219-20121201012843/@comment-3146930-20121204051338

The seven of the prophecy are called the Heroes of Olympus because the series is called The Heroes of Olympus, as it is a simple way to identify them. The seven of the prophecy are the ones that answered the call of the prophecy, but that doesn't mean that they are the only important characters in the series, as secondary characters are always important.

As far as the possible foreshadowing, the temple where the Doors of Death are located is called the House of Hades. However, what Hades is referring to in The Last Olympian is his family, as they were the ones that defended Olympus. It wouldn't make any sense at all for him to say that a temple thousands of miles away in Greece would be the savior of Olympus.