Great question! Some of the best early work on this was done by ex-Mozilla, current-Google employee Josh Carpenter - check out:
http://www.joshcarpenter.ca/declarative-3d/ http://www.joshcarpenter.ca/vr-browsing-explorations/
There's a lot we can do that is even beyond these early explorations to deliver new features to developers to experiment with and users to try soon via Servo, alongside GeckoView in new VR/AR-focused browser products.
If you take a look at the link I provided above on remaining work and rough estimates to get things just working in Servo (and not FULLY web compat), you're looking at an effort of several years for the entire team, and that's assuming the web platform both stayed still and went ahead and finished writing all the tests and specs for everything on the web that is today neither tested nor fully spec'd. I don't see that in the cards for 2018.