The Nexus 9 has taken Android Open Source to a new level making things even easier for developers. Normally when releasing a custom ROM for Android you would need the device proprietary binaries packaged in but the Nexus 9 has changed that it seems:
No proprietary binaries are needed for Volantis. The proprietary vendor binaries are on a separate ‘vendor’ partition.
Jean-Baptiste Quéru discovered this line and in fact he even worked on this when he worked on AOSP. To my understanding what this means is now the device proprietary files will always be on the device and no longer have to be packaged into /system. Those devices proprietary files will be stored on a separate vendor partition instead which won’t be wiped when installing a new ROM on the device. This makes distributing custom software much easier on devices. Now this is just on the Nexus 9 but hopefully this will become an OEM standard further increasing the Open Source world.