I commented here on Tango 3.5 yrs ago: "remains to be seen if Google can persuade cellphone manufacturers to include 2 special cameras + 2 extra processors in their future devices." Looks like that was the case.

It appears the ARCore API is well designed and 1-1 feature equivalent to ARKit, i.e. VIO + plane estimation + ambient light estimation. The API's even share a lot of names, e.g. Anchor, HitTest, PointCloud, LightEstimate.

Now that stable positional tracking is an OS-level feature on mobile, whole sets of AR techniques are unlocked. At Abound Labs (http://aboundlabs.com), we've been solving dense 3D reconstruction. Other open problems that can be tackled now include: large-scale SLAM, collaborative mapping, semantic scene understanding, dynamic reconstruction.

With Qualcomm's new active depth sensing module, and Apple's PrimeSense waiting in the wings (7 yrs old, and still the best depth camera), the mobile AR field should become very exciting, very fast.