Some time ago I had this idea for an AR game where you have a patch of land you defend from falling asteroids with a turret. The idea is that you can walk around your turret and point your phone and tap to target the asteroids.
It turned out not to be so much fun because the library I was using (Vuforia) had trouble with fast movements and background tracking when the AR marker was no longer visible. So I pivoted and moved toward a Google Cardboard VR game, which was much more fun.
Last night I spent hours tearing my hair out trying to get the new Vuforia API (in unity) to work. Even after deleting all plugins and reinstalling I still had no joy, but finally figured out that the Vuforia plugin needs to be imported first before the Google Cardboard plugin. This made it work, but the problem now is that the android "Cardboard" app no longer recognises the game as a cardboard game so it doesn't appear in the Cardboard app's "Library" tab. The game itself still works fine in VR/Cardboard mode, so I'll figure this little bug out later.
There were quite a few changes in the new Vuforia plugin, but overall the improvements are quite positive! Tracking is working quite well now when the AR marker isn't visible - as long as there is some decent background visuals and decent lighting.