Lately, I have skimmed the Percy Jackson and Heroes of Olympus books. I was looking for unfulfilled curses or any other foreshadowing Rick Riordan may have left. I have read his blog, searched Amazon books, and googled practically everything I could think of relating to HoH. This is what I have found:
1) In TLT, the oracle says You will fail to save what matters most in the end. Percy thinks this was his mom, however, he also feels uneasy. What does the oracle mean by "in the end?" In the end of the book? Series? What about failing to save Annabeth in the end of HoH? I don't actually think this is true. Rick Riordan knows that if he kills Annabeth or Percy, he will have and angry mob of fans at his house. However, it is still possible.
2) Grover will come! Rick Riordan says on his blog that we will see Grover a little bit.
3) We might see the Titans, or at least Kronos. Remember that he is in Tartarus. However, he is also in a million pieces.
4) Obviously, we will see just about every major monster that exists. I mean, this is Tartarus we are talking about! I am hoping to see Typhon. I read in the book, Greek Gods and Heroes (Which inspired Rick Riordan) about Typhon. He is the worst monster there is, basically.
These are the best predictions I have so far. I will update when I have more!