"Seven half-bloods shall answer the call,
To storm or fire the world must fall,
An oath to keep with a final breath,
And foes bear arms to the doors of death."

So we know that there are going to be at least seven new characters. We also know that Percy, Annabeth and maybe a few others from the first series will be in the second series (though they won't be main characters). Rick Riordan is trying not to reveal much information about the new series so that it will be a total surprise when it comes out, so we don't know all that much about it.

Let's look at the prophecy. We know that the oracle often means something different than what it appears to mean so the first line, "Seven half-bloods shall answer the call" seems pretty straight-forward but what is the "call"? The call of the gods, the call of the wild, it could be anything. "To storm or fire the world must fall" It doesn't sound like there is another meaning for that... "An oath to keep with a final breath" So we know someone is going to die and that there is an important oath. But what is the importance of the oath? "And foes bear arms to the doors of death". Now, "foes bear arms..." that sounds like an attack, "doors of death" Hades palace maybe? So maybe there is going to be an attack on the underworld? Now this is only what I have thought up, none of this information is definite.

Now we know there are going to be seven half-bloods. Do you think some of them will be sons and daughters of the minor gods who have finally received recognition at the camp? How many girls and how many boys are there going to be? Is the main character going to be a girl this time? I know, lots of questions and not a lot of answers, but it does start you thinking doesn't it? Well I hope we get some answers soon.