Getting a device Daydream certified is a documented process that any smartphone can apply for. Simply have a HD resolutin that can run two separate streams at 60 frames per second with a latency under 25 milliseconds.
It also needs to run without overheating and maintain a decent battery drain rate. That sounds quite simple right? Hardware is only part of the story and Google need to do its part in the Daydream experience software side of thins. It sounds that Android O is heavily focused on bringing improvements that could see more devices Daydream certified.
Specificially these changes revolve around the graphic APIs that can be used from Vulkan and OpenGL. Android O will be better at drawing data on the screen compared to previous versions of Android. This will mean that the process of taking this data and turning into a VR experience will be much more efficient.
Android O also improves the way background processes are handled so this will also benefit VR and could see more devices see that Daydream certification as battery life becomes more efficient as a result.
It seems Google are going big on VR and are putting a lot of emphasis to take the technology to the next level. The next year is certainly going to see huge improvements in the platform.