It would be cool if PDF files could get attached to compositions and (maybe) artists; and for compositions maybe images, too - like already possible for albums and artists.
From a non-technical point of view it looks like an easy thing to do because the functionality already exists for albums and doesn’t probably require much new fancy stuff. 
That way for instance scores would find a proper place within Roon. Attaching those on an album level doesn’t really help much if the actual reference is the composition and one happens to have more than one performance of that composition in the (local or remote) library.
Something along this line was requested already. Almost but not exactly the same, I think. 