Maybe it's not possible without abandoning the dumb runtime folders. 

It's a dilemma for sure, but a large part of the problem would be the support for legacy content. And the other difficulty would be that if it DOES become "open ended" there would have to be at least some sort of standard that hundreds of content developers can agree on so that those who use content would know where to find things.

Maybe one solution is to revamp the library system so that a content developer can create a folder under their own merchant name, and beneath that make a folder for each product. That single folder could contain everything ... poses, figures, add-ons, morphs, textures, and whatnot, in a single folder. Then the users can install it anywhere they like.

BUT ... and this is a big but ... when there is a reliance between products to make two products work together (morph packages, geometry calls, etc), it makes it difficult to search for things with an "open ended" library structure. We can safely assume that each vendor will arrange things in the way they think is best and easiest, but what is easy for one isn't always easy for another. On top of that, we can ALSO assume that customers will put things where they want to put them, making the "search for" possibilities even more difficult to anticipate.

It seems to me it would just be a matter of removing the reliance of having to put SPECIFIC file types in a SPECIFIC library folder in order to gain access to them. I mean, if you can make a pose file add materials, expressions, joint parameters, and other things, or change a figure to a prop by changing the file extension and hacking a couple of lines, then maybe a single file extension that can be a "does any type of content" solution would be the way to go. The addition of a Scene library could solve that, but convincing the developers to USE it is a different matter. Look at how confused everyone got when conforming figures started showing up in the Props library. 8-)

Dunno how easy that would be, though.