Last month at WWDC, Apple introduced ARKit for iOS 11. This new kit, along with native VR support in Metal 2 on macOS, was the realization of Tim Cook's hints of interest in AR. I think from here there are some guesses you can make about where they might go next. But specifically I think their next move should be to make a head mounted case for the iPhone.
Experiencing AR through a phone that you're holding at arms length can lose the illusion pretty quickly. You're looking at the screen and the world around the screen at the same time so unless you lock you arms up and hold the phone right in front of your face, it's difficult to be convinced that what you're looking at is real. The phone becomes a kind of warped prism into another place. Also, what you're looking at is 1 dimensional, so you're brain doesn't put it into the physical space the same way that it would if you could really see depth.
While any company can make VR cases for the iPhone, I think that Apple should make one to improve the overall iPhone experience. Apple could integrate the headset with iOS so that as soon as you put the phone into the case, the user is given a VR interface to explore apps with VR optimized experiences and interfaces. This would mean that you don't have to ensure the phone it in "VR mode" before placing it into a case. And as soon as you remove it from the case, you would automatically be taken back to the normal iOS interface.
Apple seems to have done a really amazing job using VIO to accurately understand the space you're in, and where the phone is in it. With a headset, the phone would be connected to a fixed position on your body which would let the software you're interacting with better understand where your face and body are in the virtual space. This would allow for things like animated characters being able to make eye contact with you or avoid running into you.
Looking at the leaks of the iPhone 8 so far, one could actually guess that Apple is already planning this. The non-Plus iPhone 8 is rumored to have two lenses which would help with spacially tracking rooms in 3D. The way that the lenses are positioned on the leaked phones also makes me think that they're optimizing for landscape viewing on the phone, an orientation that a headset would enforce.
We'll have to wait and see what Apple does. But one thing is clear - we're just at the start of mobile VR technology. This is touch screen phones in 2002. It's just a matter of increasing display quality, spacial tracking accuracy, and probably putting a phone on your face.