I think the current 26 presets for aiStandardSurface are great and I believe adding more would be even better. Does anyone else feel the same way? I imagine the only downside would be the dropdown menu become more cluttered.
That being said I'm not sure if someone has already made a sort of preset library to share.
What kind of materials you would be looking for?
Here are some ideas:
https://greyscalegorilla.com/product/metal-material-collection-volume-1/
https://greyscalegorilla.com/product/everyday-material-collection/
https://www.vray-materials.de/#Glass
https://www.vray-materials.de/#Architecture
These are just some samples that I've seen. It appears that v-ray has a very large catalog of material collections at vray-materials.de. Something similar for Arnold for Max would be beneficial, especially if Autodesk/Arnold hosted it an maintained the site.
Jim Perry linked better examples than I could have in another comment here.