As mentioned in the original announcement, Midnight at the Celestial Palace is "the first game in an all new fantasy adventure series." As in more than one game.
In fact, the first release will be labeled Midnight at the Celestial Palace: Chapter I.
Dividing the game into chapters was a decision made quite early on. After fleshing out the main characters, the setting, and figuring out what could be done with the music, we began to see how grand and epic Midnight had the potential to be. And, in many ways, needed to be. It's not just about the amount of puzzles and songs. With three separate chapters, we have the opportunity to give each act its own distinct shape and tone. Chapter I will have a very 'open' feeling, focusing on exploration and discovery while keeping a mostly lighthearted tone.
Subsequent chapters will...well, you'll just have to wait and see.