We are keeping and eye on the materials/standard surface evolution - GitHub - Autodesk/standard-surface: White paper describing Arnold's standard surface shader.
As you mentioned these formats are great for vfx / offline renders. The complex material systems can be slow to render real-time esp on WebGL. We are thinking about ways in which we could translate these complex material systems into something real-time friendly in a consistent and predictable way but it’s in the vague idea on a whiteboard phase :). It would be really useful if you could share how you envision using materialx - types of workflows etc. It can help set requirements and solidify the vague idea further.
Also for USDZ do you want to use that as a render format? I am not aware of any practical way of rendering them on the web. The only implementation I am aware of is native to iOS and uses platform specific apis not available on the web. At the moment the only way to interop with USDZ is to have transcoders to generate it and hand it off to ARQuickLook to render on iOS. I also assume most people are interested in USDZ because of ARQuickLook.
This project enables glTF to USD transcode - GitHub - kcoley/gltf2usd: command-line utility for converting glTF 2.0 models to USD
As an example of how the interop would work if you are on iOS you can check this pen -
https://codepen.io/sbtron/pen/zLYQEB
The in browser boombox is a glTF rendered through Babylon. Clicking the bottom right icon for camera (using a random image, can be anything) will launch ARQuickLook on iOS passing it the USDz version of the same boombox generated through gltf2usd.