Yeah, haptic feedback is challenging. The thing that gets challenging in my opinion with 3rd party haptic sensors is a low latency integration. Also, there are no hardware standards that define performance/latency requirements for a good AR experience. All those things will contribute to designing haptic devices.

I saw a paper recently http://aut.researchgateway.ac.nz/handle/10292/9652 around a low cost haptic game controller, which interested me a lot. There might be clever ways to create alternate haptic feedback like hot or cold sensations when an object is grabbed or interacted with (https://www.sciencedaily.com/releases/2015/11/151110082541.h...).

Because AR itself is not mainstream yet, it is quite hard to figure out what is the right haptic feedback that feels natural. I am hoping this will evolve.