The specification and the plan that looks very good. Quite clearly its a huge topic and to start with one idea is better than remain at the discussion stage forever, tough it might be wise to see it as a prototyping to learn what works good and what new ideas need to be incorporated.
What could be added to the specification would be use cases. I like what @yorik said about the user being the most difficult part. I am sure FreeCAD users will come up with a million different ways to manage their parts with that tool, e.g. in their favourite structure.
I think the user community could help the project by agreeing on a handful of key use cases than can be put to test with the earliest of implementations. I give an example.
- User A designs a CNC project and needs a linear rail system. So he wants to look at available models with <19mm rail width.
- User B designs freaky housing for an PCB project and want to look through the available potentiometers and buttons. He wants to see all parts for 6mm axis and potentiometers with threads.
- User D wants to download all FreeCAD internet library objects from the "professional line" of the manufacturer "UK Doorgrips & Co"