This already exists! Vendors often have demo boards of their parts, with complete schematics and design files, so it is pretty easy to find what you need.

Of course. The problem with these is that the license is most often best described at "it's unclear"... so if you're designing a piece of "open hardware", you can't just go and slap together stuff from implementer guides and datasheets. I mean, you can, stuff will usually work out on a technical level, but as it's not your work but someone else's you can get into trouble with copyright law.

A fully, truly open library of building blocks under actual open source licenses however? That would be a dream.