[Should we go ahead and report feature requests for the Material Library here, or in the dedicated thread?]
IIRC, the Material Library is scheduled for some minor updates (v6) including a global toggle for Real-World Scale, but the library would also benefit from some curation. You can see the Fabric collection is really impressive - many DIFFERENT materials with optimized graphs, etc.
However, the Plastic and Metal collections need a significant overhaul. I don't think there is much value in having +30 metals / plastic materials that are all the same - with the exception of the albedo / reflection color and glossiness level; these parameters are easy enough for users to adjust on their own. On the other hand, it would be extremely valuable to have a wider VARIETY of metals and plastics:
Powder coat, carbon fiber, brushed, hairline, patinated, spin brushed, parkerized, sandblasted, hammered, PVC, thermoplastic, compressed rubber, terrazzo, linoleum, kevlar twill, polysterene, fine grain, microbead grain (MOLD-TECH), etc.