Aaccurate 3-d scanning of 3d environments - houses, mines you name it. I think the tech is there in your pocket and with good software plus very accurate measurement of location, orientation + panorama/spehrical photo shoots, you could have very high quality recreations of real world areas - which in turn could be explored, modified and enjoyed in VR/AR/MR.
For example simulating house refurbishment or furniture purchases, remotely inspecting places to buy/rent/go on holiday.
The tech is mostly there, but I'm too lazy to put it altogether as I know that someone with access to more capital will also attack it, sooner or later, not just startup but IKEA, Airbnb, etc...