Guys, I have some good news to spread to you all! I think I have finally figured out what the next world is. And that is...
Don't believe me? I have some evidence.
1: If you go to "Game Worlds" in the P101 website and click "Valencia", you'll notice some screenshots are in a different place of Valencia, a place we haven't been to yet.
2: Blind Mew hinted that the new skyway for book 15 starts with the letter "C". And when you get Meowiarty to translate the stone markings written in Aquilian Linear B, the message from the Toy Maker reads this:
"I know how to defeat Kane. Find me in Valencia. Seek out Caligostro. -G"
And since we're headed for Caligostro, the skyway will most likely be called "Caligostro Skyway". So it would only make since if we were going to Valencia.
Then again, I could be wrong. We might be going to Monquista or MooShu or a new world that we haven't been to yet like Krokotopia or Grizzleheim or Polaris or so.