> A noun phrase is not necessarily a sub-category of the original noun.

This is actually something I had not fully considered. I had always considered odd modifiers to create a non-central example of the category, rather than creating a new category.

To take your example of a sperm bank, I had always considered that to be a kind of generalized version of a bank. A general "place you can deposit things for future use".

Do you have any other good examples? I'm gonna have to let this roll around in my head for a bit to see what the implications are.