User blog comment:SonOfZeus1200/The Camp Half-Blood & Kane Chronicles Series will collide!/@comment-64.134.161.179-20120528223153/@comment-64.134.161.179-20120528230159

Exactly, the HoO series will take him all the way to 2015, which means he'd release 1 book per year instead of 2 books per year as previously. If he continues to release 2 books per year, that would mean the next three HoO books as well as three new Egyptian books.

And quite frankly, everything about the last couple chapters of the Serpent's Shadow screamed "sequel series": The references to the Kanes having future troubles, the Sea of Chaos reference, the "other magic" bit, hell even the name of the last chapter (quite reminiscent of the last chapter of PJO). The only reason there wasn't an outright prophecy rhyme at the end of TSS regarding a new series like there was in TLO is because there are no rhyming prophecies in Egyptian mythology.