Cool secret addition no one knows about?

I hope there will be plenty. The beta provides a lot of room for exploration in the form of "Ooh, what's that thing over there? I'll go check it out." But I'd prefer some super secret areas and bosses.

Travel the world to get five mysterious sigil relics and turn them in to a magic stone wall in a secret, undisclosed location to reveal a map that leads to an ancient, long-forgotten secret mermaid city devoted to worship of Leviathan or something like that.