So, I have just recently finished the Son of Neptune. My first thought on it was that it was extremely more enjoyable than its predeccesor. More in-depth characters, and a better plotline. The only flaw I could find, was the relationship betwen the three protoganists. In my opinion it was not as well done as it could have been and just rushed a bit. However, this is the best book of the series so far.
Now, after reading the book, I was just in awe of the amazing character backgrounds, especially concerning the 7 demigods. You got percy, read the books and you know he has the most kick-butt story. Hazel, she defeated a giant, died, then came back to life! Frank, a chinese roman that can shapeshift! Awesome! Leo, his nana was Gaia and he's got the fire moves! Jason, praetor, fought krios, and has an epic weapon. And then Piper....who thinks her dad doesn't pay attention to her......(Piper hater here). Either way a majority of these characters are just amazing and I am really looking forward to them working together.
Now for the seventh demigod. This is my opinion. Annabeth is way to obvious of a prediction. Though it could happen, I highly doubt it. Since there are three roman demigods, and three greek demigods, I think that the seventh will be a demigod from neither camp. My first guess was Nico, as he doesn't stay much at camp Half-Blood, and isn't accepted at Camp Jupiter. But as Hazel is a daughter of Pluto, that idea went out the door for me. So this leads me to my second prediction, a child of the protogenoi. According to the books, Gaia was the first god, but earlier it states that the gods are forgetful, so the primordia;l gods, may still exist in the series. Also, the protogenoi are not recognised at either camp So this leads to my comclusion that the final demigod will be the Son/Daughter of either Chaos (The first god), Phanes (The first King of the gods), Nyx (The first Queen of the gods), or Gaia herself. It would be an interesting twist.
Well those are some of my thoughts.