Is there a "canon" of Zen Koans?
Sure - the Blue Rock Collection, the Mumonkan, the Book of Serenity, etc.  We mostly stick to those because, frankly, they're freaking fantastic and have been leading people to enlightenment for over 1200 years.  Why reinvent the wheel?  Of course, just about every teacher will often make up a koan on the spot to help a student with a specific obstacle.  Some of these remain in circulation among some lineages, but they rarely (if ever) rise to the prominence of something collected in the afore mentioned texts.